Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0155143

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DOI： 10.1063/5.0123349

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.energyfuels.2c01337

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/5.0068560

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/5.0065215

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jcim.1c00440

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0044300

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/aic.17009

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/aic.17009

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

<title>Abstract</title>
We investigate the thermodynamic stability of clathrate hydrates at cryogenic temperatures from the 0 K limit to 200 K in a wide range of pressures, covering the thermodynamic conditions of interstellar space and the surface of the hydrosphere in satellites. Our evaluation of the phase behaviors is performed by setting up quantum partition functions with variable pressures on the basis of a rigorous statistical mechanics theory that requires only the intermolecular interactions as input. Noble gases, hydrocarbons, nitrogen, and oxygen are chosen as the guest species, which are key components of the volatiles in such satellites. We explore the hydrate/water two-phase boundary of those clathrate hydrates in water-rich conditions and the hydrate/guest two-phase boundary in guest-rich conditions, either of which occurs on the surface or subsurface of icy satellites. The obtained phase diagrams indicate that clathrate hydrates can be in equilibrium with either water or the guest species over a wide range far distant from the three-phase coexistence condition and that the stable pressure zone of each clathrate hydrate expands significantly on intense cooling. The implication of our findings for the stable form of water in Titan is that water on the surface exists only as clathrate hydrate with the atmosphere down to a shallow region of the crust, but clathrate hydrate in the remaining part of the crust can coexist with water ice. This is in sharp contrast to the surfaces of Europa and Ganymede, where the thin oxygen air coexists exclusively with pure ice.

DOI： 10.3847/PSJ/abc3c0

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Other Link： https://iopscience.iop.org/article/10.3847/PSJ/abc3c0

Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/5.0021635

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jctc.9b00941

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/1.5139697

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/1.5119748

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5111843

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5096429

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5096556

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{MDPI} {AG}  

DOI： 10.3390/cryst9050248

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

We demonstrate, by molecular dynamics simulations, that water confined between a pair of insect hyperactive antifreeze proteins from the longhorn beetle Rhagium inquisitor is discontinuously expelled as the two proteins approach each other at a certain distance. The extensive striped hydrophobic–hydrophilic pattern on the surface, comprising arrays of threonine residues, enables water to form three independent ice channels through the assistance of hydroxyl groups, even at 300 K. The transformation is reminiscent of a freezing–melting transition rather than a drying transition and governs the stable protein–protein separation in the evaluation of the potential of mean force. The collectivity of water penetration or expulsion and the hysteresis in the time scale of ten nanoseconds predict a potential first-order phase transition at the limit of infinite size and provide a new framework for the water-mediated interaction between solutes.

DOI： 10.3390/cryst9040188

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/1.5083021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jpcc.8b09834

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Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/1.5044568

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jpcb.8b04441

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY  

GenIce is an efficient and user-friendly tool to generate hydrogen-disordered ice structures. It makes ice and clathrate hydrate structures in various file formats. More than 100 kinds of structures are preset. Users can install their own crystal structures, guest molecules, and file formats as plugins. The algorithm certifies that the generated structures are completely randomized hydrogen-disordered networks obeying the ice rule with zero net polarization. (c) 2017 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

DOI： 10.1002/jcc.25077

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PMID: 29212321

DOI： 10.1021/acs.jpcb.7b10581

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PMID: 29278335

DOI： 10.1021/acs.jpcb.7b10356

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.langmuir.7b01764

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

More than 300 kinds of porous ice structures derived from zeolite frameworks and space fullerenes are examined using classical molecular dynamics simulations. It is found that a hypothetical zeolitic ice phase is less dense and more stable than the sparse ice structures reported by Huang et al. [Chem. Phys. Lett. 671, 186 (2017)]. In association with the zeolitic ice structure, even less dense structures, "aeroices," are proposed. It is found that aeroices are the most stable solid phases of water near the absolute zero temperature under negative pressure. (C) 2017 Author(s).

DOI： 10.1063/1.4994757

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Clathrate hydrates of water-soluble guest molecules, such as ethylene oxide (EO) and tetrahydrofuran (THF); have been often investigated in experimental studies instead of gas hydrates because their dissociation temperatures are higher than the ice point under ambient pressure. We examine the formation mechanism of EO and THF hydrates using molecular dynamics simulations. The crystal growth rates are determined by the simulations of the hydrate/solution two-phase coexistence. It is found that the growth rate of EO hydrate is an order of magnitude higher than that of THF hydrate. The growth rates of THF hydrate largely deviate from the Wilson-Frenkel model, while the model well approximates the growth rates of EO hydrate, indicating that trapping of guest molecules on the hydrate surface, which causes the slowing of crystal growth of THF hydrate, is insignificant for EO hydrate. We also perform long-time simulations of aqueous EO and THF solutions to examine nucleation of clathrate hydrate. Spontaneous nucleation occurs only in the EO solution within the simulation time. Similar to previous studies on methane hydrate, the obtained solid structure exhibits no long-range order. It is found that the 5(12) hydrate cage, which is the most dominant cage type in the early stage of the nucleation of methane hydrate, is not a major cage type in the nucleation process of EO hydrate.

DOI： 10.1021/acs.jpcc.6b06498

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We investigate why no hydrogen-disordered form of ice II has been found in nature despite the fact that most of hydrogen-ordered ices have hydrogen-disordered counterparts. The thermodynamic stability of a set of hydrogen-ordered ice II variants relative to ice II is evaluated theoretically. It is found that ice II is more stable than the disordered variants so generated as to satisfy the simple ice rule due to the lower zero point energy as well as the pair interaction energy. The residual entropy of the disordered ice II phase gradually compensates the unfavorable free energy with increasing temperature. The crossover, however, occurs at a high temperature well above the melting point of ice III. Consequently, the hydrogen-disordered phase does not exist in nature. The thermodynamic stability of partially hydrogen-disordered ices is also scrutinized by examining the free-energy components of several variants obtained by systematic inversion of OH directions in ice II. The potential energy of one variant is lower than that of the ice II structure, but its Gibbs free energy is slightly higher than that of ice II due to the zero-point energy. The slight difference in the thermodynamic stability leaves the possibility of the partial hydrogen-disorder in real ice II.

DOI： 10.1021/acs.jpcb.5b09544

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Other Link： http://orcid.org/0000-0002-6799-6813

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Tetrahydrofuran (THF) clathrate hydrate has been frequently used in experimental studies instead of gas hydrates because it forms at a temperature higher than the ice point under ambient pressure. In this paper, we compare the crystal growth rates of THF hydrate and ice using molecular dynamics simulations. It is demonstrated that the crystal growth of THF hydrate is much slower than that of ice. The growth rates of THF hydrate significantly deviate from a standard kinetic model known as the Wilson-Frenkel model, whereas it reproduces, the temperature dependence of the growth rate of ice. The slow crystal growth and the deviation from the Wilson-Frenkel model are attributed to the trapping of THF molecules in open small cages at the hydrate surface. We calculate the free energy profile of a THF molecule transferring from the bulk solution phase to the hydrate surface using the umbrella sampling technique. It is shown that a THF molecule trapped in an open small cage needs to cross one or two free energy barriers to escape from the surface region. We also refer to the similarity between the mechanism of slow growth of THF hydrate and the effect of kinetic hydrate inhibitors.

DOI： 10.1021/acs.jpcc.5b10293

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Other Link： http://orcid.org/0000-0002-6799-6813

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

A new ice polymorph, called ice XVI, has recently been discovered experimentally by extracting the guest molecules from Ne hydrate. The ice and its filled form (clathrate hydrate) have a unique network topology which results in several interesting properties. Here we provide a theoretical method to calculate thermodynamic properties of a semiopen system in equilibrium with guest gas and thus occupancy of the guest can be varied with temperature and pressure. Experimental observations such as the disappearance of negative thermal expansivity and contraction of the host lattice upon encaging guest molecules are well reproduced, and those behaviors are elucidated in terms of the free energy of cage occupation and its temperature and pressure dependence. We propose an application of the method for preparing ice XVI to create metastable clathrate hydrates having intriguing properties with much lower occupancy of guest molecules than that at equilibrium, which otherwise cannot form.

DOI： 10.1103/PhysRevB.93.054118

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Other Link： http://orcid.org/0000-0002-6799-6813

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The emergence of homochiral domains in supercooled liquid water is presented using molecular dynamics simulations. An individual water molecule possesses neither a chiral center nor a twisted conformation that can cause spontaneous chiral resolution. However, an aggregation of water molecules will naturally give rise to a collective chirality. Such homochiral domains possess obvious topological and geometrical orders and are energetically more stable than the average. However, homochiral domains cannot grow into macroscopic homogeneous structures due to geometrical frustrations arising from their icosahedral local order. Homochiral domains are the major constituent of supercooled liquid water and the origin of heterogeneity in that substance, and are expected to be enhanced in low-density amorphous ice at lower temperatures.

DOI： 10.1103/PhysRevLett.115.197801

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Other Link： http://orcid.org/0000-0002-6799-6813

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The adsorption of guest and kinetic inhibitor molecules on the surface of methane hydrate is investigated by using molecular dynamics simulations. We calculate the free energy profile for transferring a solute molecule from bulk water to the hydrate surface for various molecules. Spherical solutes with a diameter of similar to 0.5 nm are significantly stabilized at the hydrate surface, whereas smaller and larger solutes exhibit lower adsorption affinity than the solutes of intermediate size. The range of the attractive force is subnanoscale, implying that this force has no effect on the macroscopic mass transfer of guest molecules in crystal growth processes of gas hydrates. We also examine the adsorption mechanism of a kinetic hydrate inhibitor. It is found that a monomer of the kinetic hydrate inhibitor is strongly adsorbed on the hydrate surface. However, the hydrogen bonding between the amide group of the inhibitor and water molecules on the hydrate surface, which was believed to be the driving force for the adsorption, makes no contribution to the adsorption affinity. The preferential adsorption of both the kinetic inhibitor and the spherical molecules to the surface is mainly due to the entropic stabilization arising from the presence of cavities at the hydrate surface. The dependence of surface affinity on the size of adsorbed molecules is also explained by this mechanism.

DOI： 10.1021/jacs.5b07417

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Other Link： http://orcid.org/0000-0002-6799-6813

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Two different scenarios have been proposed on the phase separation occurring in the deeply supercooled liquid water. We discuss what we can derive from our simulation results for the two scenarios and propose a way for future investigation. We also demonstrate that the phase separation in the supercooled liquid water looks like the separation of liquid water and vapor just below the conventional critical point.

DOI： 10.1103/PhysRevE.91.016302

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Publishing type：Research paper (scientific journal)   Publisher：Taylor & Francis  

DOI： 10.1080/08927022.2014.951642

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Publishing type：Research paper (scientific journal)   Publisher：The Royal Society of Chemistry  

DOI： 10.1039/c5cp03008k

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We propose a spin model compatible with ice VII-plastic ice phase transitions and critical phenomena discovered recently by computer simulations. The Blume-Capel spin-1 Ising model is extended in order to describe the entropic stabilization effect in the plastic ice phase. The model shares the same set of tricritical exponents with simulation, indicating that they are of the same universality class.

DOI： 10.1021/jp5049502

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Molecular dynamics simulations of the dissociation of methane hydrate in aqueous NaCl solutions are performed. It is shown that the dissociation of the hydrate is accelerated by the formation of methane bubbles both in NaCl solutions and in pure water. We find two significant effects on the kinetics of the hydrate dissociation by NaCl. One is slowing down in an early stage before bubble formation, and another is swift bubble formation that enhances the dissociation. These effects arise from the low solubility of methane in NaCl solution, which gives rise to a nonuniform spatial distribution of solvated methane in the aqueous phase. We also demonstrate that bubbles form near the hydrate interface in dense NaCl solutions and that the hydrate dissociation proceeds inhomogeneously due to the bubbles.

DOI： 10.1021/jp507978u

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report a molecular dynamics simulation demonstrating a fast spontaneous liquid-liquid phase separation of water and a subsequent slow crystallization to ice. It is found that supercooled water separates rapidly into low-and high-density domains so as to reduce the surface energy in the rectangular simulation cell at certain thermodynamic states. The liquid-liquid phase separation, which is about two orders of magnitude faster than the crystallization, suggests a possibility to observe this phenomenon experimentally.

DOI： 10.1103/PhysRevE.89.020301

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Other Link： http://orcid.org/0000-0002-4969-4715

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We investigate the dissociation of methane hydrate in liquid water using molecular dynamics simulations. As dissociation of the hydrate proceeds, methane molecules are released into the aqueous phase and eventually they form bubbles. It is shown that this bubble formation, which causes change in the methane concentration in the aqueous phase, significantly affects the dissociation kinetics of methane hydrate. A large system size employed in this study makes it possible to analyze the effects of the change in the methane concentration and the formation of bubbles on the dissociation kinetics in detail. It is found that the dissociation rate decreases with time until the bubble formation and then it turns to increase. It is also demonstrated that methane hydrate can exist as a metastable superheated solid if there exists no bubble.

DOI： 10.1021/jp412692d

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Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of High Pressure Science and Technology  

DOI： 10.4131/jshpreview.24.265

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

A phase behavior around the transition between ice VII and a plastic phase of water is investigated by molecular dynamics simulation and the subsequent analysis on the basis of Landau theory. The prior works have predicted that the phase transition between ice VII and plastic ice is a first-order transition on the ground of a weak hysteresis and so on. A rigorous survey in the present report, however, augments their prediction with new evidence that a first-order phase transition line gives way to a second-order one at higher pressures, where a tricritical point joins these phase boundaries together. Critical phenomena are also observed whereby, other than that associated with the hypothetical critical point in the deeply supercooled state, which could influence the physical properties in a wide range of temperatures and pressures. A new critical behavior is affirmed by the result that the scaling law holds at any pressure on the second-order phase transition line for which the critical exponents are estimated. We introduce an appropriate order parameter to obtain the Landau free energy functional and the change in the functional against temperature accounts for the phase behaviors. This also enables an estimate of the coexistence and the spinodal lines at pressures below the tricritical point, all of which compensate those obtained directly by molecular dynamics simulations. These results allow us to anticipate that the critical fluctuations may give us a clue for determining the phase boundary experimentally.

DOI： 10.1039/c3cp54726d

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>A new metastable ice phase is discovered in the freezing pathway into ice VII in accord with Ostwald's step rule.</p>

DOI： 10.1039/c4cp01616e

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

On being heated, ice melts into liquid water. Although in practice this process tends to be heterogeneous, it can occur homogeneously inside bulk ice(1). The thermally induced homogeneous melting of solids is fairly well understood, and involves the formation and growth of melting nuclei(1-5). But in the case of water, resilient hydrogen bonds render ice melting more complex. We know that the first defects appearing during homogeneous ice melting are pairs of five-and seven-membered rings, which appear and disappear repeatedly and randomly in space and time in the crystalline ice structure(6-8). However, the accumulation of these defects to form an aggregate is nearly additive in energy, and results in a steep free energy increase that suppresses further growth. Here we report molecular dynamics simulations of homogeneous ice melting that identify as a crucial first step not the formation but rather the spatial separation of a defect pair. We find that once it is separated, the defect pair-either an interstitial (I) and a vacancy (V) defect pair (a Frenkel pair), or an L and a D defect pair (a Bjerrum pair)(9) - is entropically stabilized, or 'entangled'. In this state, defects with threefold hydrogen-bond coordination persist and grow, and thereby prepare the system for subsequent rapid melting.

DOI： 10.1038/nature12190

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Institute of Physics Inc.  

Cage occupancies of hydrogen molecules in a clathrate hydrate have been examined by means of semi-grand canonical Monte Carlo simulations where hydrogen molecules enter into or leave from it in the presence of promoter species. This kind of simulation allows to evaluate the thermodynamic stability via the chemical potential of water at a given temperature and pressure. In order to make a better estimation of the chemical potential, we adopt a different standard state from the corresponding empty one. It is revealed that the present method is indeed effective to minimize errors associated with the numerical simulations.

DOI： 10.1063/1.4848090

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Authorship：Last author   Language：English   Publishing type：Part of collection (book)   Publisher：JOHN WILEY & SONS INC  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

A plastic phase predicted to exist between crystalline ice VII and liquid water has been investigated. The present work focuses on the dynamics of the water molecules in the phases at high temperature and pressure, i.e. ice VII, plastic ice, and liquid water. The hydrogen-bond correlation function providing the lifetime of hydrogen bonds is compared to the reorientational correlation function to examine a relation between a rotation of an individual molecule and an energetic relaxation process. The hydrogen-bond correlation function of plastic ice decays in a way similar to liquid but it converges to a finite value as seen in ice VII, reflecting the rotational motion of the water molecule at the fixed location. In addition, the relaxation times of the two correlation functions for plastic ice resemble one another, confirming the fact that only the rotational motion invokes the hydrogen-bond rearrangements in plastic ice.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Grandcanonical Monte Carlo simulations are performed in order to examine the cage occupancies of carbon dioxide clathrate hydrate in both the larger and the smaller cages, the latter of which has been controversial issue and is related significantly with its phase behavior. It is found that occupancy of CO2 in the smaller cage of clathrate structure I is negligible at a pressure of CO2 below 1 MPa while that in the larger cage increases gradually with increasing the pressure of CO2 in equilibrium with the clathrate hydrate. It is suggested by the present simulations that CO2 clathrate hydrate exhibits a different character in occupancy from ethane, which is a little larger than CO2 and cannot enter into the smaller cage.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We perform isothermal-isobaric molecular dynamics simulations of aqueous solutions of potassium and chloride ions with varying tube diameter. An equilibrium state is attained within 1 ns, but the relaxation time to equilibrium is dependent on the initial arrangement of ions. When the tube diameter is larger than 1 nm, ions exchange their axial positions in few tens of picosecond to be the most stable configuration in which counterions are lined up alternately. However, ions in a tube narrower than 1 nm cannot exchange the mutual positions in axial-coordinate within an order of nano second. The axial self-diffusion coefficients are also examined. It is found that they increase with increasing the tube size once an equilibrium is attained. These results suggest the partial hydration and dehydration play a certain role in diffusion of the electrolytes in confined space like tubes and channels.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan  

DOI： 10.1143/JPSJS.81SA.SA018

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We show a relevant metastable phase diagram of the polymorphs of the Frank-Kasper type clathrate hydrate by estimating the chemical potential of the second most stable phase as well as the most stable one at given thermodynamic conditions with arbitrarily chosen guest molecules.

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We present a method that brings prediction of phase behaviors of various clathrate hydrates with firm statistical mechanical ground adopting a different reference state from the usual one. Accommodation of a large guest molecule makes the frequencies of the lattice vibrational motions higher, which is one of the breakdowns of the assumptions in the original van der Waals and Platteeuw theory. The frequency modulations are incorporated in the free energy of cage occupation in the present method. Moreover, the reference state, which is originally the corresponding empty clathrate structure, is alternated to a state where cages of at least one sort are fully occupied. This meets the stability condition of clathrate hydrates that most of the cages should be accommodated. Owing to this new reference state, the thermodynamic stability is evaluated with reasonable accuracy from the free energy of cage occupation especially by a large guest molecule without considering its dependence on the cage occupancy. This conversion is also beneficial to establish a relation between the chemical potential of water and the cage occupancy from grandcanonical Monte Carlo simulation. We show a new method indeed works well in predicting the dissociation pressures of clathrate hydrates containing isobutane, propane, ethane, Xe, and CF(4).

DOI： 10.1021/jp205067v

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Language：English   Publishing type：Research paper (scientific journal)  

We have investigated structural and energetic characteristics of plastic ice, which was found in a high pressure region such as 10 GPa by molecular dynamics simulation and free energy calculation. It was predicted that plastic ice intervenes between ice VII and liquid water, in which diffusion is suppressed but rotation is allowed. In the present work, the structure in plastic ice is explored from both local and global view points and focus is placed on the local arrangement, the extent of deviation from the ideal lattice position, and the hydrogen-bonded patterns. The roles of the attractive interaction and the repulsive part of Lennard-Jones potential are also examined. It is found that the higher interaction energy in plastic ice induces a large dislocation of water molecules, which eventually conducts a facile rotation. There are a large amount of hydrogen-bonds which do not orient to the tetrahedral directions. These orientational defects give rise to fusion of the two interpenetrating sublattices of ice VII leading to a plastic phase rather than defect-containing ice VII, which results in a unique network structure of the plastic ice. © the Owner Societies 2011.

DOI： 10.1039/c1cp21871a

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Language：English   Publishing type：Research paper (scientific journal)  

Recent reports indicate that the crystallization of clathrate hydrates occurs in multiple steps that involve amorphous intermediates and metastable clathrate crystals. The elucidation of the reaction coordinate for clathrate crystallization requires the use of order parameters able to identify the reactants, products, and intermediates in the crystallization pathway. Nevertheless, existing order parameters cannot distinguish between amorphous and crystalline clathrates or between different clathrate crystals. In this work, we present the first set of order parameters that discern between the sI and sII clathrate crystals, the amorphous clathrates, the blob of solvent-separated guests and the liquid solution. These order parameters can be used to monitor the advance of the crystallization and for the efficient implementation of methods to sample the rare clathrate nucleation events in molecular simulations. We illustrate the use of these order parameters in the analysis of the growth and the dissolution of clathrate crystals and the spontaneous nucleation and growth of clathrates under conditions of high supercooling. © 2011 American Institute of Physics.

DOI： 10.1063/1.3613667

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We propose a strategic method to search for a new clathrate hydrate by investigating the selectivity of its crystalline structure, which has been expected to depend mostly on the guest species and less on the thermodynamic conditions. Relative stability among various morphologies is examined in terms of the chemical potential of water of clathrate hydrates in equilibrium with guest gases. This evaluation is performed by calculating the free energy of cage occupancy according to the standard statistical mechanical theory with the aid of the Yarmolyuk and Kripyakevich's rule on the Frank-Kasper type alloys to estimate the numbers of cages of various types. Thus, a comprehensive interpretation of the selectivity of crystalline structures is successfully made. We explain why two major structures are stable in most of the thermodynamic conditions and establish a relation between polymorphism of clathrate hydrate and the guest size and other parameters, thereby suggesting a way to find a new clathrate hydrate by appropriate choice of the guest species and/or the thermodynamic properties. It is found that there is a small room in the above parameter space for the other structure than the major two to be the most stable, including a new structure. In addition, simple but thorough elucidation is given for preferential formation of TS-I structure in bromine hydrate.

DOI： 10.1021/jp203478z

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Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/jz100340e

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Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevLett.103.017801

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.103.017801/fulltext

Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The network motif of water, vitrite, is introduced to elucidate the intermediate-range order in supercooled liquid water. Unstrained vitrites aggregate in supercooled liquid water to form very stable domain. Hydrogen bond rearrangements mostly occur outside the domain, so that the dynamical heterogeneity also stands out. Pre-peak in the structure factor of low density amorphous ice is reproduced by inter-vitrite structure factor. The vitrite can therefore be regarded as a plausible building block of the intermediate-range order and heterogeneity in supercooled liquid water and low-density amorphous ice.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Basic three-dimensional units of the network, called fragments, are introduced to characterize the hydrogen bond (HB) network structure of water. Topological differences among normal liquid water, water at low temperature, and water under high pressure are elucidated by their fragment statistics. Water at low temperature has almost defect-free network and is filled with stable fragments with small distortion. It is found that there exists a certain way on how fragments mutually aggregate. Well-formed aggregates heterogeneously constitute very stable network structures. HB network rearrangements occur scarcely inside these aggregated domains but take place in their surface areas. The heterogeneity of HB structure and rearrangement in water is thus explained in terms of the fragment structure and its rearrangements. The fragment analysis thus elucidates the intermediate-range order in water HB network. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2772627

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Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

To evaluate the relevance of treating the hydrogen bonds in liquid water as a digital (discrete) network and applying topological analyses, a framework to optimize the fitting parameters in various hydrogen bond definitions of liquid water is proposed. Performance of the definitions is quantitatively evaluated according to the reproducibility of hydrogen bonding in the inherent structure. Parameters of five popular hydrogen bond definitions are optimized, for example. The optimal choice of parameters for the hydrogen bond definitions accentuates the binary nature of the hydrogen bonding and the intrinsic network topology of liquid water, especially at the low temperature region. The framework provides a solid basis for network analyses, which have been utilized for water, and is also useful for designing new hydrogen bond definitions. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2431168

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Upon cooling, water freezes to ice. This familiar phase transition occurs widely in nature, yet unlike the freezing of simple liquids, it has never been successfully simulated on a computer. The difficulty lies with the fact that hydrogen bonding between individual water molecules yields a disordered threedimensional hydrogen-bond network whose rugged and complex global potential energy surface permits a large number of possible network configurations. As a result, it is very challenging to reproduce the freezing of 'real' water into a solid with a unique crystalline structure. For systems with a limited number of possible disordered hydrogen-bond network structures, such as confined water, it is relatively easy to locate a pathway from a liquid state to a crystalline structure. For pure and spatially unconfined water, however, molecular dynamics simulations of freezing are severely hampered by the large number of possible network configurations that exist. Here we present a molecular dynamics trajectory that captures the molecular processes involved in the freezing of pure water. We find that ice nucleation occurs once a sufficient number of relatively long-lived hydrogen bonds develop spontaneously at the same location to form a fairly compact initial nucleus. The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion, resulting in crystallization of the entire system.

DOI： 10.1038/416409a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A global nature of the potential energy surface (PES) of water clusters, (H2O)(20) and (H2O)(54) was investigated by using a reaction coordinate analysis and an annealing method. It was shown for (H2O)(64) that the successive reaction coordinates passing through low energy barriers lead to a deep minimum and the overall potential has so called a funnel structure, similar to PES of a protein folding. On the other hand, the smaller cluster (H2O)(20) has the rugged PES of a 'fragile' type, resulting from distinct bond reordering. (C) 1998 Elsevier Science B.V.

DOI： 10.1016/S0167-7322(98)00070-1

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The relation between topology and rearrangement dynamics of the hydrogen bond network (HEN) in the supercooled liquid water is investigated by using molecular dynamics (MD) calculation and examining topological indices. We have found that there is very strong correlation among certain pairs of hydrogen bonds.; HBN is shown to be represented by an ''undirected'' graph. Topology and rearrangement dynamics of HBN are then simply described in terms of the network defects and their motions. Based on this fact, a new lattice dynamic model is proposed. The model shows that spontaneous heterogeneous hydrogen bond rearrangement occurs even when the network structure is homogeneous. (C) 1996 American Institute of Physics.

DOI： 10.1063/1.471664

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Extended reference interaction site model (RISM) theoretical calculations and molecular dynamics simulation have been carried out for dilute aqueous solution of acetonitrile. Potential of mean force between two solute molecules was calculated. Two solute molecules tend to associate with each other by an attractive interaction between two negatively charged nitrogen atoms. It is found that ''bifurcated hydrogen bonds'' between a hydrogen atom on a water molecule and nitrogen atoms on acetonitrile molecules play an important role in the solute-solute interaction.

DOI： 10.1063/1.465838

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese  

We review recent advances in prediction of phase behaviors of various clathrate hydrates with firm statistical mechanical ground, in which the classical theory is extended to multiple occupation of cages and substitution of a fully occupied state for an empty hydrate structure. It is demonstrated that they indeed work effectively in predicting the dissociation pressures of clathrate hydrates containing hydrogen and some hydrocarbon molecules.

DOI： 10.4131/jshpreview.23.94

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Other Link： http://hdl.handle.net/2115/52366

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Other Link： http://hdl.handle.net/2115/52369

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Language：Japanese   Publisher：化学同人  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：English   Publisher：Physical Society of Japan  

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Language：English   Publisher：Physical Society of Japan  

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Language：Japanese   Publisher：Japan Society of Calorimetry and Thermal Analysis  

Various aspects of Water Dynamics are discussed; (1) Fluctuation and relaxation in hydrogen bond network rearrangement and their observation, (2) Mechanism of water freezing, and (3) Proton transfer in liquid water and ice.<br>Liquid water yields the intermittently collective motions accompanied with large fluctuations. Various relaxation processes associated with these collective motions in liquid water yield so-called 1/f spectra. We present the result of our analysis on a method, called 2-dimensional Raman/IR spectroscopy. This method will be able to deal with the mechanism of these intermittent collective motions.<br>Upon cooling, water freezes into ice. This process is a most familiar phase-transition, occurring in many places in nature, but has never been successfully simulated by a computer simulation. We report the first successful simulation for the pure water freezing process, which gives a molecular level picture of, particularly, how an initial nucleus is created and grows.<br>The proton transfer is a most basic reaction in chemistry and biochemistry. It is found that the mechanism of the proton transport in ice is completely different from that in liquid water. The repulsion from fourth coordinated water makes the facile proton transfer possible. The long range solvation is essential for the smooth transport of the proton in ice.

DOI： 10.11311/jscta1974.31.6

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：物性研究刊行会  

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10940102

Language：Japanese   Publisher：学会出版センタ-  

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Language：Japanese   Publisher：物性研究刊行会  

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10938528

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Language：Japanese   Publisher：物性研究刊行会  

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Other Link： http://hdl.handle.net/2433/95512

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Work type：Software  

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