Study of the structure of molecular complexes. IV. The Hartree-Fock potential for the water dimer and its application to the liquid state

Abstract

The Hartree‐Fock energy of the water dimer has been computed for 216 different nuclear configurations using the basis set given in the first paper of this series. Near the equilibrium configuration, a calculation was carried out using a large Gaussian basis set with optimized orbital exponents for the oxygen 3d ‐ and 4f ‐type and hydrogen 2p ‐ and 3d ‐type functions in order to get results close to the Hartree‐Fock limit. In the vicinity of the equilibrium configuration the mechanism for binding of the dimer is analyzed with the help of the bond energy analysis formalism. The importance of polarization (internal charge transfer), as pointed out previously by a number of authors, is clearly evident. The computed energies have been used to derive a simple analytical expression that reproduces the Hartree‐Fock potential energy surface to a high degree of accuracy. This analytical potential is compared with the empirical effective pair potentials proposed by Rowlinson and Ben‐Naim and Stillinger for the description of water in the condensed phase. We report preliminary results for Monte Carlo simulation of the liquid state of water using the computer program of Barker and Watts and our analytical Hartree‐Fock two‐body interaction potential. At the experimental mass densities we considered 27 water molecules in a cube with periodic boundary conditions at temperatures of 277, 298, and 348°K. The resulting pair correlation functions $g_{\mathrm{O–H}}^{\text{(2)}}$ (hydrogen bond distribution) and $g_{\mathrm{O–O}}^{\text{(2)}}$ (oxygen‐oxygen distribution) are in agreement with experimental data. It should be strongly emphasized that the properties are computed without any recourse to semiempirical data.