Combined Hartree-Fock quantum mechanical and molecular mechanical molecular dynamics simulations of water at ambient and supercritical conditions

Combined ab initio Hartree-Fock quantum mechanical/molecular mechanical (QM /MM) molecular dynamics simulations have been carried out for water at ambi ent and supercritical conditions. The QM/MM coupling potential is optimized for ambient water by scaling down the original MM charges in the MM potent ial model. Simulation results show that the modified QM/MM coupling potenti al behaves well in the studied water system. The evolution of the water str ucture from ambient to the supercritical conditions is studied and compared to the available experimental results. Our simulation results suggest that , at supercritical conditions, the data refined from the experiment by neut ron diffraction with the isotopic substitution technique probably overestim ate the hydrogen bonding interactions. (C) 2000 American Institute of Physi cs. [S0021-9606(00)50848-4].