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 pair-wise 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 (OH)-H-... repulsion-dispersion potential for 'polar' hydrogen atoms in some common force fields is directly related to the neglect of the (OH)-H-... promolecule energy, suggesting that inclusion of promolecular energies be explored to improve current force fields. (c) 2005 Elsevier B.V. All rights reserved.