2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/153178We present a method for calculating ab initio interatomic forces which scales quadratically with the size of the system and provides a physically transparent representation of the force in terms of the spatial variation of the electronic charge density. The method is based on a reciprocity theorem for evaluating an effective potential acting on a charged ion in the core of each atom. We illustrate the method with calculations for diatomic molecules.LaTeX, 4 pages, 4 figuresMaterials Sciencetext