A NEW DIRECT AB-INITIO DYNAMICS METHOD FOR CALCULATING THERMAL RATE CONSTANTS FROM DENSITY-FUNCTIONAL THEORY

We present a new direct ab initio dynamics methodology for calculating thermal rate constants from density functional theory (DFT). Dynamica l theory is based on a full variational transition state theory plus m ultidimensional semiclassical tunneling approximations. We have applie d this approach to the CH3+H-2-->CH4+H abstraction reaction using the BH&H-LYP method which is the combination of the hybrid Becke's half-an d-half (BH&H) method for nonlocal exchange and Lee-Yang-Parr (LYP) fun ctional for nonlocal correlation. The 6-311G(d,p) basis set was used i n these calculations. To obtain quantitative results, the classical po tential energy along the minimum energy path (MEP) was corrected eithe r by scaling to match a more accurate ab initio results for the barrie r heights or by carrying out single point calculations at selected poi nts along the MEP at a more accurate level of ab initio molecular orbi tal (MO) theory. By comparing with our previous QCISD results and expe rimental rate constants, we found that DFT particular the BH&H-LYP met hod can provide sufficient accurate potential energy surface informati on for rate calculations for this system. The present direct DFT dynam ics method can be used for reactive dynamics studies of reactions invo lving large polyatomic molecules from first principles. More work howe ver is still needed to test the accuracy of DFT methods for such calcu lations.