A Fock-operator complete active space self-consistent field (CAS-SCF) method combined with frozen-density embedding

We report the implementation of a Fock-operator complete-active space self-consistent field (CAS-SCF) method combined with frozen-density embedding (FDE) into KOALA quantum-chemistry program. The is based on configuration interaction from an unrestricted reference determinant and able to treat electronic configurations such as singlet, triplet, or quintet states embedded in molecular environment. In order account for possible spin polarization effects, FDE contribution extended case. assess convergence obtained at example stretched lithium dimer significant multi-reference character. efficiency enables orbital optimization 25 state-average SA[S0–S10,T1–T12,Q1–Q2]-CAS(10,10)-SCF calculation retinal molecule using def2-TZVP basis. ansatz leads orbitals localized by definition target system, thus facilitating selection required CAS methods complex environments.