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

Abstract

Approximate CASΠDFT scheme of the Ec calculation is considered, in which its non-dynamic Ecnd and dynamic Ecd 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 CASΠDFT closely reproduces both complete basis set (CBS) Ec limits and their trends in the series of the prototype multiple-bonded molecules C2, HCN, N2, and CO.