Akira Yoshimori

Theory of ion solvation dynamics in mixed dipolar solvents

Time dependent density functional theory in its “extended linear” or “surrogate” form is used to investigate the dynamics of selective ion solvation in binary dipolar solvents. It is shown that simple analytical approximations that trap the basic physics of the solvation process can be obtained. In particular, it is found that the relaxation of the solvent number densities about a charged solute is governed by two distinct modes clearly associated with electrostriction and redistribution processes. This is consistent with the physical picture suggested by molecular dynamics (MD) simulations. The solvent polarization relaxation is also dominated by two modes associated with the two rotational diffusion constants of the binary solvent. In addition to the analytical approximations, full numerical solutions of the extended linear theory are obtained and the dependence of the relaxation on solvent density and solute charge is discussed. Detailed comparisons of the theory with MD simulations for a closely related model indicate that the theory is qualitatively correct, but quantitatively poor generally predicting relaxation rates which are too fast. This is due mainly to the neglect of inertial or non-Markovian effects in the theoretical approach.Time dependent density functional theory in its “extended linear” or “surrogate” form is used to investigate the dynamics of selective ion solvation in binary dipolar solvents. It is shown that simple analytical approximations that trap the basic physics of the solvation process can be obtained. In particular, it is found that the relaxation of the solvent number densities about a charged solute is governed by two distinct modes clearly associated with electrostriction and redistribution processes. This is consistent with the physical picture suggested by molecular dynamics (MD) simulations. The solvent polarization relaxation is also dominated by two modes associated with the two rotational diffusion constants of the binary solvent. In addition to the analytical approximations, full numerical solutions of the extended linear theory are obtained and the dependence of the relaxation on solvent density and solute charge is discussed. Detailed comparisons of the theory with MD simulations for a closely relat...

AN INVESTIGATION OF DYNAMICAL DENSITY FUNCTIONAL THEORY FOR SOLVATION IN SIMPLE MIXTURES

Linear and nonlinear versions of time dependent density functional theory are solved for a single solute particle in a simple binary solvent. All particles interact with Lennard-Jones potentials. The theoretical results are compared with molecular dynamics calculations. It is shown that the nonlinear theory is necessary in order to obtain a good quantitative description of selective solvation dynamics. The linear theory is only of qualitative value. Also, attention is drawn to a previously little appreciated problem which arises when one attempts to compare time dependent density functional theory with computer simulation or experimental results. The difficulty involves matching the theoretical and absolute time scales and is discussed in detail in this paper.

Modeling of Spring Bloom in the Western Subarctic Pacific (off Japan) with Observed Vertical Density Structure

Effects of vertical stability on spring blooms of phytoplankton were investigated for the western subarctic Pacific ocean using a one-dimensional (depth) ecosystem model. In the model, vertical stability was expressed by diffusion constants calculated from observed density distribution. Dynamics of phytoplankton in blooms was calculated by the model using the vertical diffusion. Then, the calculated results were compared with the Coastal Zone Color Scanner (CZCS) data. The comparison shows that the shallow surface mixed layer causes early start days of spring blooms at inshore (northern) stations. In addition, spring blooms continue long at inshore (northern) stations since a water column has weak stability. This is because weak stability of a water column causes large nutrient supply from a deep layer and large diffusive transport of phytoplankton biomass from the subsurface maximum.

TIME DEPENDENT DENSITY FUNCTIONAL METHODS AND THEIR APPLICATION TO CHEMICAL PHYSICS

This article reviews microscopic development of time dependent functional method and its application to chemical physics. It begins with the formulation of density functional theory. The time dependent extension is discussed after the equilibrium formulation. Its application is explained by solvation dynamics. In addition, it reviews studies of nonlinear effects on polar liquids and simple mixtures.

Nonlinear effects of number density of solvent molecules on solvation dynamics

For the number density of solvent molecules, nonlinear effects on solvation dynamics are studied using the dynamical density functional method. The present method includes nonlinear coupling between the number density and a polarization field only in the free energy functional. By means of the nonlinear free energy functional, nonlinear differential‐integral equations are developed for the polarization field and number density. Numerical calculations show that solvent molecules relax more slowly around an ion than around a neutral solute. This result agrees qualitatively with nonlinear effects observed in many molecular dynamics simulations. In addition, the nonlinear dynamics of hydrogen bonds can be understood by considering the slow relaxation of the number density of solvent molecules.

Specific heat anomaly at the glass transition

A general frame work is devised to obtain the specific heat of nonequilibrium systems described by the energy-landscape picture, where a representative point in the phase space is assumed to obey a stochastic motion which is governed by a master equation. The specific heat depends on the observation time and becomes quenched one for short observation time and annealed one for long observation time. In order to test its validity, the frame work is applied to a two-level system where the state goes back and forth between two levels stochastically. The specific heat is shown to increase from zero to the Schottky form as the observation time is increased from zero to infinity. The anomaly of specific heat at the glass transition is reproduced by a system with a model energy-landscape, where basins of the landscape form a one-dimensional array and jump rate between adjacent basins obeys a power-law distribution. It is shown that the glass transition can be understood as a transition from an annealed to a quenc...

Theoretical Study of Dielectric Saturation in Molecular Solutions by the Monte Carlo Simulation

By using a spherical hard core model, Monte Carlo simulation study has been made on physico-chemical properties of the dielectric saturation taking place in polar solvents surrounding a charged molecule. The strength of the dielectric saturation has been evaluated by the magnitude of the free energy curvature as a function of the polarization in the radial direction. Detailed analysis is made on the problem how the dielectric saturation shell-width and its strength change, depending on the valency (magnitude of the charge) of the solute molecule and on the dipole moment of the solvent molecule as well as on the radii of solute and solvent molecules. Based on these results, we conclude that the dielectric saturation is a considerably universal phenomenon which occurs in various molecular systems.

Cooling rate dependence of specific heat in systems out of equilibrium

The anomaly of specific heat in systems out of equilibrium, especially the measurement procedure dependence of specific heat, is investigated by means of free energy landscape. Introducing measurement procedure which is based on experimental method, we propose a calculation method of specific heat in systems out of equilibrium and find an abrupt change in specific heat between annealed and quenched states. For longer observation time the change in specific heat occurs at lower temperature and becomes sharper. For slower cooling of a system the transition temperature becomes lower. This cooling rate dependence of the transition temperature is consistent with experiments and thus the abrupt change in specific heat can be regarded as the glass transition which is thermally identified.

Molecular dynamics study of fast dielectric relaxation of water around a molecular-sized ion

We have calculated the dielectric relaxation of water around an ion using molecular dynamics simulations. The collective motion of water near the ion showed fast relaxation, whereas the reorientational motion of individual water molecules does not have the fast component. The ratio of the relaxation time for the fast component and the bulk water was consistent with the experimental results, known as hyper-mobile water, for alkali halide aqueous solution.

Monte Carlo simulation study on reorganization energy of electron-transfer reactions in polar solution

Abstract The reorganization energy of electron-transfer reactions as a function of the distance between donor and acceptor molecules is calculated by Monte Carlo simulation. It is found that the reorganization energy of charge-separations reactions is appreciably different from that of charge-recombination reactions. The average of these values agrees very well with the value obtained by a mean-spherical-approximation theory.