Partial Atomic Charges, Dipole Moments and Electrostatic Potentials

Abstract

Partial atomic charges are indispensable tools in chemistry, materials science, and atomic-scale modeling of materials, yet they are not uniquely defined. In this work, we introduce a new charge-assignment model—the Dipole-Projected Overlap Density (DPOD) charges—and assess it systematically against several other schemes: Mulliken (M), Löwdin (L), electrostatic-potential-fitted (ESP), Weighted Löwdin (WL), also introduced here, and Mulliken-Dipole (MD) on which DPOD builds. Using Kohn–Sham DFT electron densities for a diverse test set of 37 molecules, we evaluate the ability of each scheme to: (i) reproduce the molecular dipole moment, (ii) accurately describe the electrostatic potential in regions relevant for intermolecular interactions, and (iii) yield chemically meaningful and transferable charge distributions. The DPOD model, which incorporates both monopole and bond-directed dipolar contributions from overlap densities and distributes intra-atomic dipoles locally, is shown to reproduce the molecular dipole moment by construction while providing a robust and chemically consistent description of atomic charges. Across the entire test set, DPOD and ESP charges are the only methods that consistently yield accurate dipole moments; however, ESP charges often suffer from weakly determined fits leading to chemically unreasonable values, particularly in hydrocarbons. DPOD charges, in contrast, remain stable across chemical environments and achieve good electrostatic-potential description. Surprisingly, WL charges, which are related to Natural Population Analysis, also display reasonably good electrostatic-potential performance despite poor dipole-moment accuracy. Overall, our results demonstrate that the DPOD scheme combines strong physical grounding, excellent dipole-moment reproduction, good electrostatic-potential accuracy, and chemical interpretability, making it a promising and reliable option for atomistic simulations, force-field parameterization, and studies requiring chemically meaningful partial atomic charges.