-DFCAS: A Hybrid Density Functional Complete Active Space Self Consistent Field Method

A multireference density functional theory method, named lambda-DFCAS, is presented in this paper. Based on the strategy used in lambda-DFVB method, the lambda-DFCAS method divides electronic correlation into two parts: the static correlation is covered by multi-reference wave function method, complete active space self-consistent field (CASSCF), while the dynamic correlation is provided by Kohn-Sham density functional theory (KS-DFT). The parameter lambda in lambda-DFCAS, controlling the hybrid extent of dynamic and static correlations, varies according to the multireference character of a given molecular system. The lambda-DFCAS method provides a multireference density functional theory with size-consistency. Test examples show that the accuracy of lambda-DFCAS is close to the complete active space second perturbation theory (CASPT2) method, while its computational cost is roughly the same as CASSCF.