Development of water model for computer simulation

The development of an intermolecular potential function of water is a fundamental topic in computer simulations because of the importance of obtaining a molecular understanding of the structure and dynamics of water for many areas of science. After more than half a century, one of main reasons that the development of a phase transferable intermolecular potential function of water is still an on-going effort is due to our inability to find a correct way to account for the many-body effects of intermolecular interaction.; As a first step toward a new intermolecular potential function of water, we determined distributed charge models of water by fitting them to both ab initio electrostatic potentials of an isolated water monomer and ab initio dimer electrostatic interaction energy. We found that the use of a diffusive charge distribution (the SQP4_Omodel) dramatically improved the model's ability to describe the electrostatic potentials of a monomer over pure point charge model, but did not adequately approximate the ab initio dimer electrostatic interaction energy. Our study suggested that in order to describe both ab initio database of monomer electrostatic potentials and dimer electrostatic interaction energy, the use of more than four diffusive charge distributions was necessary.; Based on distributed charge models derived from our first analysis, two kinds of new intermolecular potential functions of water were developed. The first model consists of the analytical two-body and three-body terms. Parameters of the two-body term were determined by fitting them to the ab initio dimer interaction energy, and a parameter of the three-body term was optimized for a density of ice-Ih. Our two-body term and three-body terms were unable to reproduce the structure of liquid water. We concluded that we needed more complex three-body functions than the Axilrod-Teller triple dipole function.; The second model is a polarizable model that changes its charge distribution according to a given electrostatic field and is implemented through the extension of the chemical potential equalization method. Parameters of the polarizable model were determined by fitting them to the ab initio dimer interaction energy, the experimental polarizability, and the experimental second viral coefficients.