N -reference, M-state coupled-cluster method: Merging the state-universal and reduced multireference coupled-cluster theories

We propose a generalization of the reduced multireference coupled-cluster method with singles and doubles (CCSD) to the genuine MR, state-universal (SU) CC approaches. Two key ingredients of this generalization are (i) the algorithm for the cluster analysis of general MR configuration interaction (CI) wave functions that is based on the SU cluster ansatz of Jeziorski and Monkhorst, and (ii) the formulation of the SU CC method employing a general (incomplete) model space. These recent developments enable us to employ modest size MR CISD wave functions that are based on an $N$ -dimensional reference space ${\mathcal{M}}_1$ as a source of higher-than-pair-cluster amplitudes in the externally corrected SU CCSD method that is based on an $M$ -dimensional model space ${\mathcal{M}}_0,$ forming a subspace of ${\mathcal{M}}_1.$ An appropriate choice of ${\mathcal{M}}_0$ and ${\mathcal{M}}_1$ makes it then possible to avoid the most severe intruder-state problems. The method is illustrated on the often-investigated H4 and H8 model systems.