Full configuration interaction potential energy curves for breaking bonds to hydrogen: An assessment of single-reference correlation methods

Several approximate correlation methods have been assessed for bond breaking reactions in BH, HF, and $CH4$ by comparison to the full configuration interaction limit. Second-order Møller–Plesset perturbation theory, coupled-cluster singles and doubles (CCSD), coupled-cluster with perturbative triples [CCSD(T)], and the hybrid density-functional method Becke three parameter Lee–Yang–Parr have been considered. Both restricted and unrestricted references have been used along with the basis sets aug-cc-pVQZ, 6-31G^**, and 6-31G^* for BH, HF, and $CH4$ respectively. Among the methods considered, unrestricted CCSD and CCSD(T) provide potential energy curves which are the most parallel to the benchmark full CI curves, but the nonparallelity errors are fairly large (up to 6 and 4 kcal mol⁻¹, respectively). Optimized-orbital coupled-cluster methods provide superior results but nevertheless exhibit approximately the same maximum errors.