A comparison of the renormalized and active-space coupled-cluster methods:: Potential energy curves of BH and F2

The renormalized noniterative CCSD(T) and CCSD(TQ) methods, which are the examples of the method of moments of coupled-cluster (CC) equations, and the active-space CC approach with internal and semi-internal triexcited clusters (CCSDt) are applied to the potential energy curves of BH and F-2. It is shown that the renormalized CCSD(T) and CCSD(TQ) methods are practically as effective in removing the failing of the standard noniterative CC approaches at larger internuclear separations as the CCSDt method. The results of the CCSDt and renormalized CCSD(T) and CCSD(TQ) calculations are also compared with those obtained with other perturbative CC approaches that focus on bond breaking. (C) 2001 Elsevier Science B.V. All rights reserved.