Multistructural partition function truncation and its effect on the thermal rate constants

Thermal rate constants for the hydrogen abstraction reaction of methyl pentanoate were calculated using the multistructural canonical variational theory with small-curvature tunneling (MS-CVT/SCT). The conformational search for the stationary points generated by these reactions was performed with an algorithm that combines systematic and stochastic searches at dual-level. At the high-level (MPWB1K/6-31+G(d,p)), 244 geometries for methyl pentanoate and transition states were found. A systematic estimative of the efficiency of the truncation of multistructural rovibrational partition functions and its effects on thermal rate constants was carried out. In this analysis, we observed that rotation about -OCH₃ group (ϕ₄) of the ester generates unstable structures with little contribution to the magnitude of the partition function. We also estimate that the truncation of the partition functions in a small set of conformations can produce deviations up to 75% compared to converged theoretical results.