A new direct ab initio dynamics method for calculating thermal rate constants from density functional theory

Abstract

We present a new direct ab initio dynamics methodology for calculating thermal rate constants from density functional theory (DFT). Dynamical theory is based on a full variational transition state theory plus multidimensional semiclassical tunneling approximations. We have applied this approach to the CH₃+H₂→CH₄+H abstraction reaction using the BH&H‐LYP method which is the combination of the hybrid Becke’s half‐and‐half (BH&H) method for nonlocal exchange and Lee–Yang–Parr (LYP) functional for nonlocal correlation. The 6‐311G(d,p) basis set was used in these calculations. To obtain quantitative results, the classical potential energy along the minimum energy path (MEP) was corrected either by scaling to match a more accurate ab initio results for the barrier heights or by carrying out single point calculations at selected points along the MEP at a more accurate level of ab initio molecular orbital (MO) theory. By comparing with our previous QCISD results and experimental rate constants, we found that DFT particular the BH&H‐LYP method can provide sufficient accurate potential energy surface information for rate calculations for this system. The present direct DFT dynamics method can be used for reactive dynamics studies of reactions involving large polyatomic molecules from first principles. More work however is still needed to test the accuracy of DFT methods for such calculations.