Combining Multiconfigurational Wave Functions with Density Functional Estimates of Dynamic Electron Correlation. 2. Effect of Improved Valence Correlation

We present further tests of a recently introduced scheme ( J. Phys. Chem . 1996, 100, 10131) for combining multiconfigurational wave functions with density functional estimates of the dynamic electron correlation. We investigate the equilibrium geometries and bond dissociation energies for selected bonds in a series of 25 molecules at the CASSCF level and also with the new method (which we term CASΔDF). The CASΔDF method approximately halves the error in R e and D e with respect to the CASSCF level. We have also investigated the role of improving the valence correlation through extension of the active space used in the CASSCF reference wave function. Again the CASΔDF method produces a halving of the mean errors in R e and D e with respect to the CASSCF level.