Improving upon CCSD„T...: 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 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. 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, C2, N2, and H2O are reduced from 22 mhartree for CCSD T to 10 mhartree for CCSD T . Using CCSD 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 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. © 2008 American Institute of Physics. DOI: 10.1063/1.2830236