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 CASΠDFT 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 CASΠDFT 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.