We propose a size-consistent method to combine small active space multiconfigurational self-consistent-field (MCSCF) wave functions with standard correlation energy density functionals. The correlation energy is not evaluated from the standard spin densities but from a pair of alternative densities obtained from the natural orbitals and occupation numbers. The method substantially improves the MCSCF estimates of the spectroscopic constants of a set of 11 diatomics, with an accuracy comparable to that from Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid functional and Becke–Lee-Yang-Parr (BLYP) functional spin-unrestricted Kohn-Sham density functional theory (DFT) calculations. The method also provides estimates in good agreement with multireference coupled-cluster calculations for the diradical-involved automerization barrier of cyclobutadiene, with deviations $\sim 0.3-1.7\mathrm{kcal}{\mathrm{mol}}^{-1}$ as compared to deviations $\sim 14–15\mathrm{kcal}{\mathrm{mol}}^{-1}$ provided by B3LYP or BLYP spin-restricted Kohn-Sham DFT calculations. It also yields rather good estimates of energy differences between triplet and open-shell singlet states in the helium atom and the methylene molecule.

• Received 24 August 2006

DOI:https://doi.org/10.1103/PhysRevA.75.012503