Block correlated coupled cluster theory with a complete active-space self-consistent-field reference function: the...

Block correlated coupled cluster (BCCC) theory with a complete active-space self-consistent-field (CASSCF) reference function is presented. This theory provides an alternative multireference coupled cluster framework to describe the multireference characters of the ground-state wave functions. In this approach, a multireference block is defined to incorporate the nondynamic correlation, and all other blocks involve just a single spin orbital. The cluster operators are truncated up to the four-block correlation level, leading to the BCCC4 scheme. For a single bond breaking problem, the present CAS-BCCC4 approach with a CASSCF(2,2) reference function computationally scales as the traditional single-reference coupled cluster singles and doubles. We have applied the present approach to investigate the electronic structures of several model systems including H4, P4, and BeH(2), and the single bond breaking processes in small systems such as F(2), HF, BH, and CH(4). A comparison of our results with those from full configuration interaction calculations shows that the present approach can provide quantitative descriptions for all the studied systems. The size-consistency error is found to be quite small in the dissociation limit of diatomic molecules F(2), HF, and BH.