The state-universal multi-reference coupled-cluster theory with perturbative description of core-virtual excitations

The idea of combining the state-universal multi-reference coupled-cluster s ingles and doubles method (SUCCSD) with the multi-reference many-body pertu rbation theory (MRMBPT) is tested by approximating the cluster amplitudes o f the SUCCSD formalism that carry only core and virtual orbital indices by their first-order MRMBPT estimates. The resulting SUCCSD(1) approach offers considerable savings in the computer effort, while providing the results t hat are as good as those obtained with the original SUCCSD method. Test cal culations are performed for the lowest two totally symmetric singlet states of the H4 model system and methylene. (C) 2001 Elsevier Science B.V. All r ights reserved.