Improving upon CCSD(T): ΛCCSD(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 ΛCCSD(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. ΛCCSD(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, C2, N2, and H2O are reduced from 22mhartree for CCSD(T) to 10mhartree for ΛCCSD(T). Using ΛCCSD(T) with a Brueckner reference provides the best single reference coupled-cluster description of N2’s dissociation curve to date. Application of CCSD(T) and ΛCCSD(T) to the concerted transition state of 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX) indicates that this tran...