AB-INITIO BASED CLASSICAL ELECTROSTATIC POTENTIALS FOR THE INTERACTION BETWEEN MOLECULES AND SURFACES

Molecular dynamics simulations and free energy calculations based on i nteractions described by hybrid quantum-classical methods are a logica l approach to the study of solvent-molecule and solvent-surface interf aces. These techniques exploit the classical nature of long-range inte ractions to provide a computationally tractable description of complex interfacial systems. The present work develops and tests the performa nce of a classical electrostatic potential for describing adsorbate/su rface interactions. The electrostatic potential is constructed from a numerical representation of the electric potential, field, and field g radients of the bare surface, as a function of position above the surf ace, combined with the electric multipolar moments of the adsorbate mo lecules. The former is obtained from periodic Hartree-Fock calculation s on the bare surface; the latter is from molecular Hartree-Fock calcu lations on the isolated molecules. An empirical, short-range repulsive term is added to the classical ''long-range'' expression for the inte raction. The resulting potential is then tested by comparison to perio dic Hartree-Fock calculations on the combined adsorbate-surface system . This allows the accuracy of the classical approximation to be evalua ted independent of the level of theory used to compute the electronic charge densities. Results are presented for H2O, HCl, and NH3 adsorbat es on the (001) surface of MgO.