Dual-level reaction-path dynamics (the III approach to VTST with semiclassical tunneling). Application to OH + NH₃ → H₂O + NH₂

We consider a new approach to reaction-path dynamics calculations in which the reaction path is calculated at a low level (LL) of theory and stationary point information from a high level (HL) of theory is used to interpolate corrections to energetic quantities, vibrational frequencies, and moments of inertia. Such a calculation is labeled XIIIY, where X denotes the high level and Y the low level. The theory is applied to the reaction OH + NH₃ and three isotopomeric analogs. The highest-level optimization reported for the saddle point is QCISD(T)//MP2/aug-cc-pVTZ, which yields a classical barrier height of 3.65 kcal/mol. The rate constant is calculated at two levels, QCISD(T)//MP2/aug-cc-p VTZ[MP2/aug-cc-pVDZ]///MP2/6-31G and QCISD(T)//MP2/aug-cc-pVTZ[MP2/aug-cc-pVDZ]///PM3-SRP; the calculated rate constant for the unsubstituted reaction is approximately invariant to the low level used in the dual-level scheme and agrees with experiment within a factor of 1.13 at 250 K and within a factor of 1.14 at 1500 K.