Yasuaki Hiwatari

MD study of the mixed alkali effect in terms of the potential surface in the lithium-potassium metasilicate glass

Interception of jump path among the unlike alkali ion sites has been found in a previous molecular dynamics study of lithium-potassium metasilicate ((Li,K)SiO3) glass. The cause for the interception leading to the mixed alkali effect has been studied in terms of the potential surfaces. A site mismatch between the unlike ions with respect to the potential energies has been clearly observed. The activation energies of the jumps have been evaluated. The jump path of each kind of alkali ions has been visualized and the jump motions in such a path have been described from a standpoint of the mixed alkali effect.

Conformational transitions of a diblock polyampholyte in 2 and 3 dimensions

Abstract We report here the first extensive study of the conformational transitions of a diblock polyampholyte (made of two symmetric blocks, one positively, the other negatively charged). Results are obtained by means of a Monte Carlo simulation on a square (resp. cubic) lattice in 2 (resp. 3) dimensions, using the Bond Fluctuation algorithm together with a Multiple Histogram method. In both dimensions, an early (i.e. high temperature) transition is observed which is shown to correspond to the pairing of both blocks together, resulting in a `super-coil, each `super-monomer being a dipole made of one positive and one negative monomer. Phenomenological characteristics of this pairing transition are given and its specificity is outlined. A second transition is also observed at a lower temperature in both dimensions. However, whereas it is a mere coil-globule transition of the `super-coil in three dimensions, it happens to be rather a folding in two dimensions because of topological constraints.

Classical molecular dynamics for the formation process of a fullerene molecule

Abstract Molecular dynamics (MD) simulation has been used to study the formation process of the cage-shaped molecule, fullerenes, starting with atomic configurations of monocyclic, bicyclic, and tricyclic rings, which have been proposed by experiment to be intermediate atomic configurations in the formation of fullerene molecules. It has been found that the most efficient temperature for the formation of the cage-shaped structure from a ring is about 3000 K. The rotational effect has been studied as the kinetic factor in the formation process of fullerene. It has been found that the C 60 molecule is more stable than C 70 with respect to such rotation. It was also observed that the disintegration process of the molecules took place through the defects.

Study on the β Peak of χ″ for a Two-Dimensional Supercooled Fluid State Via Molecular Dynamics Simulation

Abstract We have computed the generalized susceptibility (imaginary part of χ) of two-dimensional supercooled fluid states through molecular dynamics simulations with different system sizes. It is found that when the temperature of the system is higher than Tc (critical temperature) there is no significant difference between the large (10,000-particle) and small (100-particle) systems in the χ obtained, but for much lower temperatures the spectra of the χ″ obtained reveal a remarkable system-size dependent behavior such as the β peak becomes much broader for the large system. This phenomenon is physically understood in terms of the strongly correlated motion of atoms extending over a wide range of space of the system. We will discuss in this work about the relaxations on two-dimensional supercooled fluids and their microscopic origins as well as their system-size dependence.

Slow dynamics in supercooled fluids

Abstract Molecular dynamics simulation is a useful tool in general to investigate dynamical properties of condensed matters from a microscopic point of view. This is the case for highly supercooled liquids and the liquid-glass transition as well. However, we have to pay special attentions to the length of time as well as the time-mesh size and the system size of the simulations for the study of slow dymamics which governs the relaxation of such particular systems. In this paper, we study the slow dynamics of a highly supercooled fluid by calculating the general susceptibility χ(q,ω) with both two- and three-dimensional binary soft-sphere models and super-long-time molecular dynamics simulations. It is found that the Cole-Cole analysis clearly distinguishes between the α and β relaxations of supercooled fluid states. A spurious sound-mode peak appearing in χ(q,ω) (imaginary part of the general susceptibility) turns out to be located at ω inversely proportional to the length of the simulation cell size and seems to be physically meaningless. This peak may be caused by the periodic boundary conditions used in the present simulation.

Structural Transformations of Ice at High Pressures Via Molecular Dynamics Simulations

Abstract A simple classical model is used for the study of the structural transformations of ice under high pressures, such as ice VIII to VII and X, via classical molecular dynamics (MD) simulation. In the present MD simulation, pair potentials of a simple form between pair of atoms and a thee-body potential representing the H-O-H angle dependence, originally developed by Kawamura et al., were used. Starting with a stable ice VIII at low pressure and low temperature, we have carried out two different MD runs, one with increasing pressure keeping the temperature constant (simulation I) and the other with increasing temperature under constant pressure (simulation II). From these MD simulations we have obtained the structural transformations from ice VIII to VII for both simulations; the former was finally transformed into ice X for the simulation I. The present results are compatible with recent experiments on high pressure ices.