Using the Dawson-March transformation, the Dirac density matrix y(r,r') can be written in terms of the density amplitude root rho(r) and a phase theta(r), where rho(r) gamma(r, r) is the ground-state electron density. Here, for systems such as LiH or H-3(-), it is shown how the force -partial derivative V(r)/partial derivative r corresponding to the one-body potential V(r) can be written, given the ground-state density rho(r) from a high-level ab initio calculation or a diffraction experiment. The Hartree-Fock ground-state densities for LiH and H-3(-) are utilized to calculate approximate phase angles theta(r). Finally, for H-3(-) the force -partial derivative V(r)/partial derivative r corresponding to V(r) is calculated. (c) 2006 Elsevier B.V. All rights reserved.