Complete active space coupled-cluster method. Extension of single-reference coupled-cluster method using the CASSCF wavefunction

Abstract

It is proposed that the calculation of non-dynamical electron correlation effects by the complete active space self-consistent field (CASSCF) method can be extended by the inclusion of dynamical correlation effects through the use of a suitably modified single-reference coupled-cluster (CC) procedure. The CASSCF method supplies the reference configuration and the complete set of the CC amplitudes corresponding to the space of active orbitals. These amplitudes are then fed into the CC procedure to determine the CC amplitudes of low rank, but corresponding to the full orbital space. The proposed CASCC method combines the formal simplicity of the single-reference CC method with the flexibility of the CASSCF approach, but is limited to cases where there is a single configuration with the highest weight in the CASSCF wavefunction.