The two faces of static correlation [ Article ]

 

Joshua W. Hollett^a) and Peter M. W. Gill^b)

 

• Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia

• ^a)Electronic mail: jhollett@rsc.anu.edu.au.

  ^b)Electronic mail: peter.gill@anu.edu.au.

   

   

   

  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 H₂ 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.
   
  https://sci-hub.se/https://aip.scitation.org/doi/10.1063/1.3570574