Analyzing effects of strong electron correlation within Kohn-Sham density-functional theory

A density-functional-theory method for analyzing effects of strong electron correlation is presented, based on a single Kohn-Sham determinant. It yields the population of effectively unpaired (odd) electrons and depicts the strength of nondynamic correlation, both locally and globally. It provides also a quantitative estimate of localized magnetic moments without invoking symmetry-breaking procedures. Preliminary tests on some exemplary systems of strong correlation such as C${}_2$, Cr${}_2$, the NO dimer, and dissociating H${}_2$ and N${}_2$ are discussed in comparison with available post-Hartree-Fock wave-function studies. We show that the bond in C${}_2$ is unlikely to have diradical character in its ground state, but may have it in some excited state. The singlet ground state of the NO dimer, however, does have a diradical character of the bonding. Quite interestingly, the bond in Cr${}_2$ has a quad-radical nature.

Figure 1

(a) RKS dissociation energy curves of three singlet states of C${}_2$ calculated with RI-B05/g3large using the MOM method. (b) Odd-electron atomic population vs the C-C distance for three singlet states of C${}_2$ calculated with RI-B05/g3large RKS using the MOM method.

Figure 2

Population of odd electrons in NO dimer calculated with RI-B05/g3large RKS method.