In this work, we prove that an intramolecular dielectric model yields accurate results for the forces between nonoverlapping molecules, at first order in the intermolecular interaction. The analysis is valid within the Born-Oppenheimer approximation. Within any perturbed molecule, a nonlocal dielectric function epsilon(nu)(-1)(r,r(')) describes the screening of external potentials due to the induced redistribution of electronic charge, i.e., this function acts as the integral kernel that determines the effective potential at point r (within linear response), when an external potential phi(ex)(r(')) acts on the molecule, at other points r('). The dielectric function epsilon(nu)(-1)(r,r(')) depends on the nonlocal charge-density susceptibility, which can be calculated ab initio or by density functional techniques. From quantum mechanical perturbation theory, at first order the interaction energy of two molecules is determined by the unscreened Coulomb interaction energy of the unperturbed molecular charge distributions. Yet the first-order forces on the nuclei include dielectric screening effects, due to the redistribution of the electronic charge density of each molecule in the presence of the other. This counterintuitive result follows from a relation between the charge-density susceptibility and the derivatives of the electronic charge density with respect to nuclear coordinates. The derivation provides a quantum mechanical validation for dielectric screening models on the nanoscale, when the dielectric function for electronic response is nonlocal. (C) 2003 American Institute of Physics.