Ab Initio Based Classical Electrostatic Potentials for the Interaction between Molecules and Surfaces

Molecular dynamics simulations and free energy calculations based on interactions described by hybrid quantum-classical methods are a logical approach to the study of solvent−molecule and solvent−surface interfaces. These techniques exploit the classical nature of long-range interactions to provide a computationally tractable description of complex interfacial systems. The present work develops and tests the performance of a classical electrostatic potential for describing adsorbate/surface interactions. The electrostatic potential is constructed from a numerical representation of the electric potential, field, and field gradients of the bare surface, as a function of position above the surface, combined with the electric multipolar moments of the adsorbate molecules. The former is obtained from periodic Hartree-Fock calculations on the bare surface; the latter is from molecular Hartree−Fock calculations on the isolated molecules. An empirical, short-range repulsive term is added to the classical “long-range” expression for the interaction. The resulting potential is then tested by comparison to periodic Hartree−Fock calculations on the combined adsorbate−surface system. This allows the accuracy of the classical approximation to be evaluated independent of the level of theory used to compute the electronic charge densities. Results are presented for H 2 O, HCl, and NH 3 adsorbates on the (001) surface of MgO.