Applications of multiconfigurational coupled-cluster theory

A coupled‐cluster method which permits the use of multiconfiguration reference states has recently been developed in this laboratory. In the present work, it is applied to several states of ${\mathrm{H}}_2{\mathrm{\hspace{0.17em}(}}^1{\Sigma }_g^{+}\mathrm{),}$ $\mathrm{Li}{(}^2\mathrm{S),}$ ${\mathrm{HeH}}_2{(}^1{A}_1\mathrm{),}$ and ${\mathrm{CH}}_2{(}^3{B}_1{,}^1{A}_1\mathrm{),}$ which include both open and closed shells. These applications are made within an approximation in which the cluster operator (T) is truncated at $T_2,$ ${\mathrm{T\simeq T}}_1{\mathrm{+T}}_2$ and the expansion of $e^{\mathrm{-T}}{\mathrm{He}}^T$ is truncated at the double‐commutator level. For cases where a single configuration function ceases to be a good starting point, it is found that a single configuration based truncated coupled‐cluster procedure may exhibit serious difficulties. In such cases we find it possible to choose a multiconfigurational reference state for which our coupled‐cluster procedure converges reasonably rapidly. This paper contains several illustrations of such convergence characteristics.