water
GenIce-core:Efficient Algorithm for Generation of Hydrogen-Disordered Ice Structures
Ice is different from ordinary crystals because it contains randomness, which means that statistical treatment based on ensemble averaging is essential. Ice structures are constrained by topological rules known as the ice rules, which give them unique anomalous properties. These properties become more apparent when the system size is large. For this reason, there is a need to produce a large number of sufficiently large crystals that are homogeneously random and satisfy the ice rules.
On the role of intermolecular vibrational motions for ice polymorphs. III. Mode characteristics associated with negative thermal expansion.
It is well known as an unusual property of liquid water that when it is cooled down, it begins to expand at a temperature below 4 degree celsius. When it is cooled down to 0 degree, it becomes ice, and after that, its volume becomes smaller as it is cooled down. However, even after it becomes ice, if the temperature is kept very low, it begins to expand again at an absolute temperature of 60 K or lower.
On the anomalous homogeneity of hydrogen-disordered ice and its origin
In hydrogen-disordered ice, each water molecule is oriented in a different way, and the interaction between one water molecule and the surrounding water molecules can be attractive (low interaction energy) or repulsive (high interaction energy). The interaction between a water molecule and a large number of surrounding water molecules seems to be most influenced by the orientation of the water molecules in the immediate vicinity. However, calculations and experiments have shown that the interaction between a water molecule and all surrounding water molecules is almost the same regardless of whether the direction of the nearby water molecule is attractive or repulsive.
Novel Algorithm to Generate Hydrogen-Disordered Ice Structures
A new paper from our group has been published. There is a growing demand for computer simulations of ice, which is composed of a large number of molecules, to study the behavior of molecules dissolved in small amounts in ice and to search for new crystal structures. Unlike ordinary crystals, the water molecules in ice crystals are not oriented in the same way. In order to handle ice in computer simulations, it is necessary to generate crystal structures with randomized molecular orientations in an appropriate manner.
Cage occupancy and dissociation enthalpy of hydrocarbon hydrates
An elaborated statistical mechanical theory on clathrate hydrates is applied to exploration of their phase equilibria and dissociation enthalpies. The experimental dissociation pressures of methane, ethane, acetylene, and propane hydrates are well recovered by the method we have proposed. We estimate water/hydrate and hydrate/guest two‐phase coexisting conditions in the temperature, pressure, and composition space in addition to three‐phase equilibrium conditions. It is shown that the occupancy of guest molecules and the two‐phase boundaries in the phase diagram vary depending sensitively on its size.
Lennard-Jones Parameters Determined to Reproduce the Solubility of ions
Most classical nonpolarizable ion potential models underestimate the solubility values of NaCl and KCl in water significantly. We determine Lennard-Jones parameters of Na+, K+, and Cl- that reproduce the solubility as well as the hydration free energy in dilute aqueous solutions for three water potential models, SPC/E, TIP3P, and TIP4P/2005. The ion–oxygen distance in the solution and the cation–anion distance in salt are also considered in the parametrization. In addition to the target properties, the hydration enthalpy, hydration entropy, self-diffusion coefficient, coordination number, lattice energy, enthalpy of solution, density, viscosity, and number of contact ion pairs are calculated for comparison with 17 frequently used or recently developed ion potential models.
On the role of intermolecular vibrational motions for ice polymorphs II:\ Atomic vibrational amplitudes and localization of phonons in ordered and disordered ices
We investigate the vibrational amplitudes and the degree of the phonon localization in 19 ice forms, both crystalline and amorphous, by a quasi-harmonic approximation with a reliable classical intermolecular interaction model for water. The amplitude in the low pressure ices increases with compression, while the opposite trend is observed in the medium and high pressure ices. The amplitude of the oxygen atom does not differ from that of hydrogen in low pressure ices apart from the contribution from the zero-point vibrations.