WFX Molecular Fragment References for Hirshfeld Charge-Transfer Analysis in Non-Covalent Complexes

In this work, we introduce and assess a fragment-based Hirshfeld scheme in which molecular WFX files of isolated fragments are used to reconstruct reference densities directly. The electron density of the full complex is reconstructed from its WFX file and partitioned using either atomic WFX references or molecular WFX fragment references on the same integration grid. The method is applied to water clusters and to representative complexes from the S66 set, including hydrogen-bonded, donor-acceptor, $\pi$ -stacked, symmetric, and dispersion-dominated systems. Across density-functional calculations and the available CCSD correlated-wave- function results, molecular WFX fragment references lead to substantially smaller fragment-charge magnitudes than atomic references. For hydrogen-bonded and donor-acceptor complexes, the reduction commonly reaches 75%-90% in the primary 6-311++G** calculations, whereas symmetric or dispersion-dominated systems exhibit zero or nearly zero net integrated fragment charge. The results show that atomic-reference fragment charges include contributions associated with reconstructing each molecular fragment from isolated atoms, whereas WFX fragment references measure redistribution relative to already formed isolated monomers. The proposed approach therefore provides a chemically meaningful density-based reference for analyzing intermolecular charge transfer in non-covalent complexes.