Spin-Adapted Multi-Reference Coupled Cluster Formalism Including Non-Linear Terms and its Application to the H4 Model System

TLDR: In this paper, a spin-adapted multi-reference linear coupled cluster (MRCC) formalism was extended by including quadratic terms in both the direct and the coupling part of the MRCC equations.

Abstract: Recently developed explicit form of a spin-adapted multi-reference linear coupled cluster (MRCC) formalism for the two-dimensional model space involving closed-shell-type configurations [B. Jeziorski and J. Paldus, J. Chem. Phys. 88, 5673 (1988)] has been extended by including quadratic terms in both the direct and the coupling part of the MRCC equations. The formalism has been applied to the H4 model system studied earlier by Jankowski and Paldus [Intern. J. Quantum Chem. 18, 1243 (1980)]. In the quasidegenerate region, where the single reference LCCSD approximation breaks down, the two-reference LCCSD method performs very well providing good results for both states. However, in the non-degenerate region, the linear multireference approach is plagued with intruder state problems, since the second reference state interacts strongly with other excited configurations. The inclusion of the quadratic terms in the MRCC equations resolves the intruder state difficulty. We also find that the MRCC equations possess multiple solutions capable of describing not only the two lowest states but also various other pairs of states, as long as they contain a significant contribution from the reference configurations.