Shinichi Miura

A unified scheme for ab initio molecular orbital theory and path integral molecular dynamics

We present a general approach for accurate calculation of chemical substances which treats both nuclei and electrons quantum mechanically, adopting ab initio molecular orbital theory for the electronic structure and path integral molecular dynamics for the nuclei. The present approach enables the evaluation of physical quantities dependent on the nuclear configuration as well as the electronic structure, within the framework of Born–Oppenheimer adiabatic approximation. As an application, we give the path integral formulation of electric response properties—dipole moment and polarizability, which characterize the changes both in electronic structure and nuclear configuration at a given temperature when uniform electrostatic field is present. We also demonstrate the calculation of a water molecule using the present approach and the result of temperature and isotope effects is discussed.

A molecular dynamics study of sub- and supercritical water using a polarizable potential model

A series of molecular dynamics calculations for water has been carried out along an isochore at 1 g/cm3 and an isotherm at 600 K in order to examine microscopic properties of water in the sub- and supercritical states. A polarizable potential model proposed by Dang (RPOL model) was employed to take into account the state dependence of intermolecular interaction. Along the isochore, fluid structure changes from tetrahedral icelike structure at room temperature to simple-liquidlike one at high temperatures. Orientational correlation between a tagged molecule and its neighbors is reduced substantially with increasing temperature, though hydrogen bonds between two molecules persist even at 600 K. As temperature increases, the number of the hydrogen bonds per molecule decreases monotonically from 3.2 at 280 K to 1.9 at 600 K. The activation barrier for diffusion at 600 K is about half as large as that at room temperature. A collective polarization relaxation loses collective character above the temperature whe...

An ab initio path integral molecular dynamics study of double proton transfer in the formic acid dimer

Double proton transfer in the formic acid dimer has been investigated with Car–Parrinello ab initio molecular dynamics calculations. The electronic structure of the dimer has been obtained using gradient-corrected density functional theory based on the B-LYP (Becke exchange [Phys. Rev. A 38, 3098 (1988)] and Lee–Yang–Parr correlation [Phys. Rev. B 37, 785 (1988)] functional. The optimized equilibrium and saddle-point geometries, obtained by simulated annealing, are in good agreement with previous ab initio quantum chemical predictions and experiment. Thermal and quantum fluctuations of nuclei along the double proton transfer reaction path have also been investigated at T=300 K. Thermal fluctuations give a broad distribution of nuclei around the minimum energy path on the potential energy surface. Quantum fluctuations, investigated using ab initio path integral molecular dynamics, make the distribution even broader around the equilibrium structure, and cause the distribution to deviate appreciably from the...

Ab initio molecular orbital calculation considering the quantum mechanical effect of nuclei by path integral molecular dynamics

Abstract We present an accurate calculational scheme for many-body systems composed of electrons and nuclei, by path integral molecular dynamics technique combined with the ab initio molecular orbital theory. Based upon the scheme, the simulation of a water molecule at room temperature is demonstrated, applying all-electron calculation at the Hartree–Fock level of theory.

A molecular dynamics study of dielectric constant of water from ambient to sub- and supercritical conditions using a fluctuating-charge potential model

Molecular dynamics calculations have been performed based upon a fluctuating-charge model TIP4P-FQ over a wide range of state points from ambient to sub- and supercritical conditions in order to investigate dielectric properties of water. The TIP4P-FQ could successfully reproduce the experimental dielectric constant. The dielectric constant is dominated not only by the intermolecular orientational correlation but also by the magnitude of each molecular dipole moment. Interesting behavior of these two contributions is reported as a function of temperature and density. Averaged number of hydrogen bonds has also been found to have a strong correlation with the dielectric constant.

Path integral molecular dynamics for Bose–Einstein and Fermi–Dirac statistics

In this paper, we propose a promising extension of the path integral molecular dynamics method to Bose–Einstein and Fermi–Dirac statistics. The partition function for the quantum statistics was rewritten in a form amenable to the molecular dynamics method with the aid of an idea of pseudopotential for the permutation of particles. Our pseudopotential, here, is a rigorous one describing the whole effect of Bose–Einstein and Fermi–Dirac statistics. For a model calculation, we chose a system consisting of three independent particles in a one-dimensional harmonic well. The calculation has been performed for the particles obeying Bose–Einstein and Fermi–Dirac statistics. The calculated kinetic and potential energies were in excellent agreement with the analytical results even near the ground state. It was found that the pseudopotential shows attractive and repulsive characters for the static properties of Bose–Einstein and Fermi–Dirac particles, respectively. For interacting model particle systems, we studied ...

A molecular-dynamics study of the equation of state of water using a fluctuating-charge model

Abstract The equation of state (EOS) and critical constants are reported for a fluctuating-charge model of water, based on PρT data from a series of molecular dynamics (MD) calculations. Since unphysical divergence of the partial charges sometimes occurred in the course of MD calculations, the model (TIP4P-FQ) was slightly modified to avoid this erroneous behavior. The EOS was determined by fitting the calculated PρT data to a model EOS. The calculated critical constants are in good agreement with the experimental values.

A molecular approach to quantum fluids based on a generalized Ornstein–Zernike integral equation

In this paper, we present an Ornstein–Zernike-type integral equation applicable to quantum fluids. This integral equation was obtained by averaging fully imaginary-time-dependent reference interaction site model integral equation for the quantum fluids over imaginary time. The resulting integral equation is a scalar integral equation for linear response correlation function. The self-correlation function in the integral equation was determined in a self-consistent manner with the aid of Feynman’s variational perturbation method. Our theoretical treatment is an extension of the theory for an excess electron in the classical solvents [J. Chem. Phys. 81, 1975 (1984)] to that for the fully quantum fluids. Numerical calculations have been performed for the fluid helium-4 assuming Boltzmann statistics. The calculated pair correlation functions are in good agreement with path integral molecular dynamics results. The experimental static structure factors are well described by our theory. It was found that the cal...

A generalized Ornstein–Zernike integral equation study of atomic impurities in quantum fluids

In this paper, solvation structure and thermodynamic properties of rare gas and alkali impurities in liquid helium-4 have been studied theoretically. A generalized Ornstein–Zernike integral equation for pure quantum fluids [J. Chem. Phys. 114, 7497 (2001)] was extended to the quantum solutions at infinite dilution. Self-correlation function of the solute atom which appears in the integral equation was determined self-consistently with the solvent density fluctuation. Numerical calculations have been performed for the helium-4 solutions at 4 K, with Boltzmann statistics being assumed. It was found that all the rare gas species investigated in this study have negative partial molar volumes, owing to the well-defined solvation structure around the impurities. In contrast to this, the alkali atoms have large positive partial molar volumes, primarily coming from the excluded volume contribution. Further, while the rare gas atoms have negative excess chemical potentials, the alkali atoms have large positive val...

Path integral molecular dynamics method based on a pair density matrix approximation: An algorithm for distinguishable and identical particle systems

In this paper, the path integral molecular dynamics (PIMD) method has been extended to employ an efficient approximation of the path action referred to as the pair density matrix approximation. Configurations of the isomorphic classical systems were dynamically sampled by introducing fictitious momenta as in the PIMD based on the standard primitive approximation. The indistinguishability of the particles was handled by a pseudopotential of particle permutation that is an extension of our previous one [J. Chem. Phys. 112, 10 116 (2000)]. As a test of our methodology for Boltzmann statistics, calculations have been performed for liquid helium-4 at 4 K. We found that the PIMD with the pair density matrix approximation dramatically reduced the computational cost to obtain the structural as well as dynamical (using the centroid molecular dynamics approximation) properties at the same level of accuracy as that with the primitive approximation. With respect to the identical particles, we performed the calculatio...