Coupled-cluster and configuration-interaction approaches to quasidegeneracy

The capability of the variational and perturbative-type approaches to the many-electron correlation problem-as represented by the configuration interaction (CI) and coupled cluster (CC) theories-to describe, respectively, the nondynamic and dynamic correlation effects, is emphasized, and its exploitation in the design of the so-called externally corrected CC methods, as well as in the formulation of Davidson-type corrections that are based on the CC theory, at both single reference and multireference levels, is reviewed. The performance of various methods of this type is illustrated on the DZP H4 model that consists of two interacting and slightly stretched hydrogen molecules in a trapezoidal geometry. This often studied model enables a continuous transition from the degenerate to the nondegenerate regime by varying the degree of quasidegeneracy via. a single geometric parameter. In this way the role of higher-than-pair clusters, particularly in the presence of intruder states, can be explored and the performance of various approaches that exploit the complementarity of the CI and CC approaches can be evaluated.