In CASSCF, the total set of molecular orbitals is divided into three subsets: inactive orbitals, active orbitals, and virtual orbitals. The active orbitals are those most relevant to the chemical process being studied. The method involves iteratively optimizing the orbitals and the CI coefficients within the active space until self-consistency is achieved. This dual optimization allows CASSCF to account for both dynamic and static correlations more effectively than methods that treat these aspects separately.
