The multireference coupled-cluster method in Hilbert space: An incomplete model space application to the LiH molecule

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Abstract
The first results from a Hilbert space, multireference coupled‐cluster (CC) method in an incomplete model (active) space are reported for the five lowest states of LiH. The active space is spanned by several configurations at the level of single and double excitations, where the configuration(s) causing intruder state problems are excluded from the complete Hilbert reference space. Full inclusion of single‐ and double‐excitation operators is considered in the expansion for the cluster operator, with all quadratic terms in the renormalization part. The multireference CC results for the ground (X 1Σ+) and four low‐lying excited states (a 3Σ+, A 1Σ+, a 3Π, A 1Π ) of LiH are compared with the corresponding full configuration‐interaction (FCI) energies. The agreement between FCI and CC values within a few hundredths of mH for the Π states proves the feasibility of the present method to describe, quantitatively, the quasicomplete reference space problem. Deviations of the incomplete multireference results from the exact FCI energies for for excited Σ states vary with respect to the quasidegeneracies involved from a few tenths of mH for the nondegenerate (near‐equilibrium) geometries to several mH in the dissociation region.