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

Abstract

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( 3 P) ? C 2 H 4 and singlet PES near the ÁCH 2 CH 2 OÁ biradical were extensively explored at several levels of theory. In this work, the lowest-lying O( 1 D) ? C 2 H 4 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 CH 2 CHOH ? 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 CH 3 C:-OH ? H 2 CCO ? 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.

Theoretical kinetics of O + C2H4

“…[11,15,20,23] and the discussions therein), and a new set of calculations was carried out here. The resulting energies are shown in Fig.…”

Analytic Gradients for the Spin-Flip ORMAS-CI Method: Optimizing Minima, Saddle Points, and Conical Intersections

“…At the transition state, the molecule exhibits C 2 symmetry with a dihedral angle of 92.8°a t the CASSCF level of theory. Once again, the lack of single excitations in the SF-CAS method results in some minor inaccuracies in the TS geometriesparticularly concerning the dihedral angle which SF-CAS underestimates compared with CASSCF (8,7).…”

“…Minima on a PES are characterized by a Hessian (matrix of energy second derivatives) that is positive-definite (all positive eigenvalues). These structures rarely pose a computational challenge for ordinary molecules since their electronic structure is usually well described by single-reference methods. , Transition states, on the other hand, are characterized by one negative eigenvalue of the Hessian matrix, and often they require multireference methods for an accurate representation. − This is due to strong correlation that arises from degenerate or near degenerate molecular orbitals present in these geometries. In the ethylene molecule, for example, while the minimum energy D 2h geometry is well represented by a π 2 closed shell configuration, the internally rotated transition state of D 2 d symmetry requires at least two determinants, since the π and π* orbitals approach degeneracy during the rotation to the 90° twisted geometry .…”

Atom-Based Strong Correlation Method: An Orbital Selection Algorithm

“…However, the HF bonding and lone pair occupied orbitals frequently prove unsuitable for a number of reasons. For example, a subsequent MCSCF energy optimization might lead to occupied active orbitals that include correlating orbitals outside the MBS in an improper manner. − As another example, when a particular minimum energy pathway (MEP) is studied, well-chosen MCSCF active orbitals might prove to be insufficiently flexible , to produce the correct qualitative features across the entire MEP. Furthermore, it is well-known that the HF energy optimization cannot provide suitable unoccupied orbitals of the MBS-type.…”