Electrostatic interactions based upon floating basis ab initio calculations. The water pentamer

Electrostatic interactions derived from floating basis HF/D95** ab initio calculations are presented for a tetrahedral water pentamer. Since floating basis ab initio calculations approximately satisfy the Hellmann-Feynman theorem, indicating that the forces on the nuclei can be calculated classically; and, since the wave function of the pentamer includes all polarization and mutual polarization effects, the intermolecular interactions can be calculated classically by using molecular properties extracted from the wave function of the aggregate. We compare classical interactions based upon floating basis functions, as well as analogous interactions based upon normal (nonfloating) Hartree-Fock calculations with supermolecular interaction energies and pairwise interactions. We also compare classical electrostatic calculations based upon interacting point charges with those based upon interactions of charges one molecule with the electric field of the others. We show that the latter method (which is free of penetration effects) coupled with the floating basis wave functions gives the supermolecular energy to 0.2 kcal/mol. The electrostatic interactions derived from the ''normal'' wave functions are significantly too strong. Point charge interactions for the ''normal'' wave functions are closer to the supermolecular interaction due to fortuitous. approximate cancelation of the effects of penetration and the overestimation of the electrostatic interaction. We also show that the dipole moments of the possible dimers cannot be derived additively from the monomers, nor those of the pentamer from those of either the monomers or the dimers.