An Assessment of the Accuracy of Multireference Configuration Interaction (MRCI) and Complete-Active-Space Second-Order Perturbation Theory (CASPT2) for Breaking Bonds to Hydrogen

Complete-active-space self-consistent field (CASSCF), complete-active-space second-order perturbation theory (CASPT2), and two restricted active-space variants of multireference configuration interaction (singles, doubles, and limited triples and quadruples, CISD[TQ], and second-order configuration interaction, SOCI) have been assessed for bond-breaking reactions in BH, HF, and CH 4 by comparison to the full configuration interaction limit. These results allow one, for the first time, to ascertain typical errors for such reactions across the entire potential energy curve. They also provide an assessment of different prescriptions for choosing an active space. A valence active space and a one-to-one active space were considered along with the basis sets cc-pVQZ, 6-31G**, and 6-31G* for BH, HF, and CH 4, respectively. The valence active space performs better than the one-to-one active space for BH but is inferior for HF. Always choosing the larger of the two active spaces for a given molecule leads to the best results. When using the larger of the two active spaces, the nonparallelity errors for CASPT2, CISD[TQ], and SOCI were less than 3.3, 1.4, and 0.3 kcal mol -1, respectively. These results are superior to those of unrestricted coupled-cluster with perturbative triples [UCCSD(T)] for these same systems.