Improving upon CCSD(T): Λ CCSD ( T ) . I. Potential energy surfaces

Despite the successes of spin-restricted coupled-cluster singles, doubles, and perturbative triples [CCSD(T)], it fails for systems away from equilibria, which might raise questions about transition states, e.g. In an attempt to remedy this failure, we implemented Λ CCSD ( T ) in a general purpose form for open and closed shells and with arbitrary single determinant reference functions, and applied it to a wide variety of bond-breaking examples. Λ CCSD ( T ) is shown to substantially improve the behavior of CCSD(T) at long bond lengths without significantly affecting the quality of the equilibrium results. Weighted average nonparallelity errors for HF, C 2 , N 2 , and H 2 O are reduced from 22 mhartree for CCSD(T) to 10 mhartree for Λ CCSD ( T ) . Using Λ CCSD ( T ) with a Brueckner reference provides the best single reference coupled-cluster description of N 2 's dissociation curve to date. Application of CCSD(T) and Λ CCSD ( T ) to the concerted transition state of 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX) indicates that this transition state is well described by either methods, and indicates that the activation barrier is too high for it to be a major pathway of decomposition.