title: The two faces of static correlation.
abstract: Restricted Hartree-Fock (RHF) and UHF wavefunctions for beryllium-like ions with nuclear charge 3 ≤ Z ≤ 5 are found using a near-complete Slater basis set. The triplet (RHF → UHF) instability and correlation energy are investigated as a function of Z and we find that the instability vanishes for Z > 4.5. We reproduce this surprising behavior using a minimal-basis model and, by comparing with the stretched H(2) molecule, conclude that "static" (also known as nondynamical, near-degeneracy, first-order, or strong) correlation comes in two flavors: one that can be captured by UHF and another that cannot. In the former (Type A), there is an "absolute near-degeneracy"; in the latter (Type B), there is a "relative near-degeneracy." This dichotomy clarifies discussions of static correlation effects.
authors.affiliation: Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia. jhollett@rsc.anu.edu.au
authors.forename: Joshua W
authors.initials: JW
authors.lastname: Hollett
authors.forename: Peter M W
authors.initials: PM
authors.lastname: Gill