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 $\Lambda \mathrm{CCSD}\left(\mathrm{T}\right)$ 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. $\Lambda \mathrm{CCSD}\left(\mathrm{T}\right)$ 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, ${\mathrm{C}}_2$ , ${\mathrm{N}}_2$ , and ${\mathrm{H}}_2\mathrm{O}$ are reduced from $22\mathrm{mhartree}$ for CCSD(T) to $10\mathrm{mhartree}$ for $\Lambda \mathrm{CCSD}\left(\mathrm{T}\right)$ . Using $\Lambda \mathrm{CCSD}\left(\mathrm{T}\right)$ with a Brueckner reference provides the best single reference coupled-cluster description of ${\mathrm{N}}_2$ ’s dissociation curve to date. Application of CCSD(T) and $\Lambda \mathrm{CCSD}\left(\mathrm{T}\right)$ 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.