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 H2O, HCl, and NH3 adsorbates on the (001) surface of MgO.