GENERALIZED MOLECULAR MECHANICS INCLUDING QUANTUM ELECTRONIC-STRUCTURE VARIATION OF POLAR-SOLVENTS - II - A MOLECULAR-DYNAMICS SIMULATION STUDY OF WATER

By employing the truncated adiabatic basis set (TAB) description devel oped in the preceding article [B. D. Bursulaya and H. J. Kim, J. Chem. Phys. 108, 3277 (1998), preceding paper], solvent water under an ambi ent condition is studied via a molecular dynamics (MD) computer simula tion method. The evolving charge distribution of each water molecule i s described by the mixing of the TAB functions, which fluctuates with its local environment. The parametrization of these basis functions is couched in terms of the complete active space self-consistent field ( CASSCF) ab initio calculations in vacuum. By using an interaction site representation for the diagonal and overlap charge distributions of t he basis functions, electronic polarizability both in and out of the w ater molecular plane is accounted for. The ground-state charge distrib ution for the entire solvent system is determined at the self-consiste nt field (SCF) level with a numerical iteration method. Two different models, TAB/10 and TAB/10D, are studied. The average water dipole mome nt in liquid is found to be 2.58 D for the former and 2.65 D for the l atter, while it is 1.85 D in vacuum for both models. The solution-phas e electronic polarizability distributions, characterized by a narrow b ut finite width, show that nonlinear hyperpolarizability makes a non-n egligible contribution to instantaneous electronic response of water e ven though its average response mainly falls in a linear regime. It is found that the TAB water predictions for structural, dynamic, spectro scopic, dielectric, and transport properties are in good agreement wit h corresponding experimental results. (C) 1998 American Institute of P hysics.