Performance of the spin-flip and multi-reference methods for bond-breaking in hydrocarbons: A benchmark study
Benchmark results for spin-flip (SF) coupled-cluster and multi-reference (MR) methods for bond-breaking in hydrocarbons are presented. The non-parallelity errors (NPEs) of potential energy curves are analyzed for (i) the entire range of nuclear distortions from equilibrium to the dissociation limit; and (ii) in the intermediate range (2.5-4.5 Angstrom), which is the most relevant for kinetics modeling. For methane, the spin-flip and MR results are compared against full configuration interaction (FCI). For the entire potential energy curves, the NPEs for the SF model with single and double substitutions (SF-CCSD) are slightly less than 3 kcal/mol. Inclusion of triple excitations reduces the NPEs to 0.32 kcal/mol. The corresponding NPEs for the MR-CI are less than 1 kcal/mol, while those of multi-reference perturbation theory are slightly larger (1.2 kcal/mol). The NPEs in the intermediate range are smaller for all the methods. The largest errors of 0.35 kcal/mol are observed, surprisingly, for EOM-SF(2,3), while MR-CI, CASPT2, and SF-CCSD curves are very close to each other and are within 0.1-0.2 kcal/mol from FCI. For a larger basis set, the difference between MR-CI and CASPT2 is about 0.2 kcal/mol, while SF-CCSD is within 0.4 kcal/mol from MR-CI. For the C-C bond breaking in ethane the results of the SF-CCSD are within 1 kcal/mol from MR-CI for the entire curve, and within 0.4 kcal/mol in the intermediate region. The corresponding NPEs for CASPT2 are 1.8 and 0.4 kcal/mol, respectively. Including the effect of triples by energy-additivity schemes is found to be insignificant for the intermediate region. For the entire range of nuclear separations, sufficiently large basis sets are required to avoid artifacts at small internuclear separations.