Multiconfigurational Effects on the Density Coherence

This
paper is a study of density coherences in multiconfiguration
self-consistent field theory and Kohn–Sham (KS) density functional
theory. We visualize and compare the nondiagonal elements of the first-order
reduced density matrix in the electronic coordinate representation.
This is the electronic density coherence, and it also appears in Hartree–Fock (HF) theory
in the integrand of the exchange integral. The density coherence is
calculated as a function of the internuclear distance for three diatomic
molecules (H2, F2, and HF) using both restricted
and unrestricted KS and HF theory, as well as the complete active
space self-consistent field method. We identify a group of closely
associated peaks corresponding to the coherence of electrons on opposite
sides of the center of the molecule, and we call this the exchange
massif. We find that Slater-determinant methods with a higher percentage
of HF exchange tend to underestimate the density coherence at the
exchange massif. We explain the trends in terms of a multireference
diagnostic and the number of unpaired electrons.