The use of the promolecular charge density to approximate the penetration contribution to intermolecular electrostatic energies

The electrostatic interaction energy between molecules is commonly approximated using a finite expansion over atomic multipoles, although it underestimates the exact electrostatic energy. The Coulombic energy between promolecular charge densities (a sum over pairwise electrostatic energies between spherical atoms) is an excellent approximation to the missing (penetration) energy, which is often large and can dominate the exact electrostatic energy. The use of a different O⋯H repulsion-dispersion potential for 'polar' hydrogen atoms in some common force fields is directly related to the neglect of the O⋯H promolecule energy, suggesting that inclusion of promolecular energies be explored to improve current force fields.