Block correlated coupled cluster method with a complete-active-space self-consistent-field reference function: The formula for general active spaces and its applications for multibond breaking systems

The block correlated coupled cluster (BCCC) theory is developed for a general complete-active-space (CAS) self-consistent-field reference function. By truncating the cluster operator up to the four-block correlation level, we derive the spin orbital formulation of the CAS-BCCC4 approach. The CAS-BCCC4 approach is invariant to separate unitary transformation within active, occupied, and virtual orbitals. We have implemented the approach and applied this approach to describe the potential energy surfaces for bond breaking processes in C-2 and N-2 and for a simultaneous double bond dissociation in H2O. Numerical results show that the CAS-BCCC4 approach provides quite accurate descriptions for the entire dissociation process in each of the studied systems. The overall performance of the present approach is found to be better than that of the internally contracted multireference configuration interaction singles and doubles or complete-active-space second-order perturbation theory. The size-extensivity error is found to be relatively small for N-2. (C) 2008 American Institute of Physics.