Based on the exact solutions for a non-Born–Oppenheimer (nBO) Hooke-Coulomb model for a four-particle system (J. Chem. Phys. 2005, 123, 024102), we present here nBO nuclear and electron one-particle densities for this system. We examine the effect on the topology of the one-particle densities that arises from the selection of different reference points for the densities. In addition, we analyze the behavior of these densities as a function of the mass ratio between light and heavy particles. We conclude from these results that molecular structure is not univocally related to the topology of the one-particle densities in a nBO regime. © 2012 Wiley Periodicals, Inc.