Electron correlation energy with a combined complete active space and corrected density-functional approach in a small basis versus the reference complete basis set limit: A close agreement

Approximate CAS Pi DFT scheme of the E-c calculation is considered, in which its non-dynamic E(c)(nd )and dynamic E-c(d) components are calculated with the complete active space (CAS) configuration interaction (CI) for valence bonds in a small basis and density functional theory (DFT) in the same basis, respectively. The DFT contribution is corrected for the suppression of dynamic correlation (SDC) with non-dynamic correlation. The present CAS Pi DFT closely reproduces both complete basis set (CBS) E-c limits and their trends in the series of the prototype multiple-bonded molecules C-2, HCN, N-2, and CO.