Formulating reduced density gradient approaches for noncovalent interactions

This work elucidates several forms of reduced electron density gradient (RDG) to describe non-covalent interactions (NCIs). By interpreting the RDG as a local-moment function, we systematically leveraged Weizacker's and Fermi's local moments. This resulted in high-fidelity RDG representations consistent with the NCI analysis. In addition, the RDG version derived from the Lagrangian kinetic energy density is conveniently normalized. These results suggest the non-existence of a particular RDG formulation when performing NCI analysis. Thus, an in-depth examination of the theoretical foundations connecting the RDG function with the nature of non-covalent interactions is necessary.