Moving on strongly correlated potential surfaces: Are there alternatives to CAS-SCF?

by Pulay, Peter; Bofill, Josep M.

The accurate treatment of reactive potential energy surfaces remains largely unsolved. DFT is inexpensive but works only for milder cases. Generally, Complete Active Space treatment is needed in first order. The main cost of CAS-SCF is usually orbital optimization. Fractionally occupied natural orbitals of UHF wavefunctions approx. the CAS-SCF active space well, leading to UNO-CAS which avoids orbital optimization. UNO-CAS energies are a few kcal/mol above CAS-SCF but the two surfaces are closely parallel. Anal. forces can be readily calcd. for UNO-CAS (JCP 1989 , 90, 3637). It shares, however, three problems of CAS-SCF: (1) An MC-SCF wavefunction without dynamical correlation is unbalanced. Methods of including dynamic correlation will be compared to CAS-PT2. (2) The active space can possibly change on the surface. (3) For large active spaces, the CI treatment becomes expensive. Corresponding orbital techniques can be used to reduce the configurational space drastically, exploiting the pairwise nature of correlation.