QUANTUM AND STATISTICAL MECHANICAL STUDIES OF LIQUIDS .3. DERIVING INTER-MOLECULAR POTENTIAL FUNCTIONS FOR THE WATER DIMER FROM AB-INITIO CALCULATIONS

An intermolecular potential function for the water dimer is developed from ab initio molecular orbital calculations with a minimal basis set. This allows the thorough study of the basis set dependence of Monte Carlo simulations of liquid water. General techniques for generating intermolecular potential functions are presented. A procedure for increasing the occurrence of dimer configurations with low energy when they are selected in a random fashion is discussed and utilized. A careful analysis of the forms of potential functions for the water dimer reveals several interesting observations. First, it is found that the quality of fits using 12-6-3-1 potentials is the same as for functions with exponentials (αe-βr) for the short-range interactions. However, the 12-6-3-1 functions reduce the computation times for condensed-phase simulations by 11%. It is further established that models for a water monomer with four point charges are preferable to alternatives with three charges. Distinct improvement in the predicted geometry and dimerization energy for the linear water dimer are obtained in this way. Testing procedures including searches for unwanted minima in potential functions are applied. The final four point charge 12-6-3-1 potential is found to provide an excellent representation of the potential surface for the water dimer from STO-3G calculations. In all, 291 points on the surface were considered. The standard deviations for the fit of the 12-6-3-1 function are ca. 0.3 kcal/ mol. © 1979, American Chemical Society. All rights reserved.