O(³P) + C₂H₄ Potential Energy Surface: Study at the Multireference Level

Abstract

The O((3)P) + C(2)H(4) reaction provides a crucial, initial understanding of hydrocarbon combustion. In this work, the lowest-lying triplet potential energy surface is extensively explored at the multiconfiguration self-consistent field (MCSCF) and MRMP2 levels with a preliminary surface crossing investigation; and in cases that additional dynamical correlation is necessary, MR-AQCC stationary points are also determined. In particular, a careful determination of the active space along the intrinsic reaction pathway is necessary; and in some cases, more than one active space must be explored for computational feasibility. The resulting triplet potential energy surface geometries mostly agree with geometries from methods using single determinant references. However, although the selected multireference methods lead to energetics that agree well, only qualitative agreement was found with the energetics from the single determinant reference methods. Challenges and areas of further exploration are discussed.

O + C2H4 potential energy surface: excited states and biradicals at the multireference level

“…CASSCF stationary point searches for species in the reactions described above employed analytic gradients and central differenced, numerical Hessians. As in the first paper [1], second-order, Gonzalez-Schlegel (GS2) [30], intrinsic reaction coordinate (IRC) calculations identified minima associated with each TS.…”

“…Table 3 contains energetics at several levels of theory: (1) the barriers from single-point MRMP2 energies along the CASSCF IRCs; (2) optimized MR-AQCC barriers for paths 12 and 13; (3) Nugyen et al [6] and Yang et al [14] barriers; (4) single-point MR-AQCC barriers along the CASSCF IRC for paths 12 and 13; and (5) the barriers from single-point CR-CC(2,3) energies along the CASSCF IRCs. While the single-point energies along the CASSCF IRC geometries are not strictly barriers, they should approximate the barriers [1]. Differences between single-point MRMP2 data at the MCSCF minima and maxima are labeled stationary point MRMP2 (SPMRMP2) energy barriers and always include ZPE (except for species with ORF issues in numerical Hessians as discussed below).…”

“…Since the multireference approach for these calculations has already been explained for the triplet surface [1], only a summary of that procedure is given here. To recover the majority of the quasi-degenerate correlation, all calculations were carried out at the full optimized reaction space (FORS) [20][21][22], or complete active space self-consistent field (CASSCF), level of theory using the aug-cc-pVTZ [23] basis set.…”

“…To recover the majority of the quasi-degenerate correlation, all calculations were carried out at the full optimized reaction space (FORS) [20][21][22], or complete active space self-consistent field (CASSCF), level of theory using the aug-cc-pVTZ [23] basis set. The previous paper [1] compared calculations between aug-cc-pVTZ and augcc-pVDZ [23] and showed the double zeta basis set to be sufficient for this system. The determinant-based method [24] and the full Newton-Raphson converger with the augmented Hessian technique [25][26][27] were used because of the large complete active space (CAS) sizes and the biradical species.…”

“…ethylene reaction, both the two lowestlying triplets and two lowest-lying singlets play important roles in the reactions of the initially formed biradicals and the ensuing dynamics. Our initial study [1] of the lowestlying triplet PES examined pathways 1-9 from the Nguyen et al [6] work as well as an additional path 10. The current study expands this earlier work [1] by examining multireference barriers for the lowest-lying singlet paths [10][11][12][13]20 as labeled in Nguyen et al [6], excited states for path 1, the Miyoshi path (M), and important surface crossings.…”