Mapped Interpolation Scheme for Single-Point Energy Corrections in Reaction Rate Calculations and a Critical Evaluation of Dual-Level Reaction Path Dynamics Methods

Three procedures for incorporating higher level electronic structure data into reaction path dynamics calculations are tested. In one procedure, variational transition state theory with interpolated single-point energies, which is denoted VTST-ISPE, a few extra energies calculated with a higher level theory along the lower level reaction path are used to correct the classical energetic profile of the reaction. In the second procedure, denoted variational transition state theory with interpolated optimized corrections (VTST-IOC), which we introduced earlier, higher level corrections to energies, frequencies, and moments of inertia are based on stationary-point geometries reoptimized at a higher level than the reaction path was calculated. The third procedure, called interpolated optimized energies (IOE), is like IOC except it omits the frequency correction. Three hydrogen-transfer reactions, CH3 + H‘H → CH3H‘ + H (R1), OH + H‘H → HOH‘ + H (R2), and OH + H‘CH3 → HOH‘ + CH3 (R3), are used to test and validate the procedures by comparing their predictions to the reaction rate evaluated with a full variational transition state theory calculation including multidimensional tunneling (VTST/MT) at the higher level. We present a very efficient scheme for carrying out VTST-ISPE calculations, which are popular due to their lower computational cost. We also show, on the basis of calculations of the reactions R1−R3 with eight pairs of higher and lower levels, that VTST-IOC with higher level data only at stationary points is a more reliable dual-level procedure than VTST-ISPE with higher level energies all along the reaction path. Although the frequencies along the reaction path are not corrected in the IOE scheme, the results are still better than those from VTST-ISPE; this indicates the importance of optimizing the geometry at the highest possible level.