O+C2H4 potential energy surface: lowest-lying singlet at the multireference level

In previous studies (West et al. in J Phys Chem A 113(45): 12663, 2009; West et al. in Theor Chem Acc 131: 1123, 2012), the lowest-lying O(P-3) + C2H4 and singlet PES near the center dot CH2CH2O center dot biradical were extensively explored at several levels of theory. In this work, the lowest-lying O(D-1) + C2H4 PES is further examined at the multiconfigurational self-consistent field (MCSCF), MRMP2, CR-CC(2,3), GVB-PP, and MR-AQCC levels. This study aims to provide a detailed comparison of these different levels of theory for this particular system. In particular, many reactions for this system involve multiple bond rearrangements and require various degrees of both non-dynamic and dynamic correlation for reasonable energetics. As a result of this variety, coupled cluster results parallel but do not always match up with multireference results as previously anticipated. In the case of the (CHCHOH)-C-2 -> oxirane pathway, MCSCF results show the possibility of a two-step mechanism rather than an elementary step, but the case is very difficult to elucidate. In the case of the CH3C:-OH -> H2CCO + H-2 pathway, a non-traditional NEB MEP at the GVB-PP level and MR-AQCC stationary point determination illustrate the need for a complex treatment of this surface.