Transition state theory: Variational formulation, dynamical corrections, and error estimates

摘要

Transition state theory (TST) is revisited, as well as evolutions upon TST such as variational TST in which the TST dividing surface is optimized so as to minimize the rate of recrossing through this surface and methods which aim at computing dynamical corrections to the TST transition rate constant. The theory is discussed from an original viewpoint. It is shown how to compute exactly the mean frequency of transition between two predefined sets which either partition phase space (as in TST) or are taken to be well-separated metastable sets corresponding to long-lived conformation states (as necessary to obtain the actual transition rate constants between these states). Exact and approximate criterions for the optimal TST dividing surface with minimum recrossing rate are derived. Some issues about the definition and meaning of the free energy in the context of TST are also discussed. Finally precise error estimates for the numerical procedure to evaluate the transmission coefficient kappa(S) of the TST dividing surface are given, and it is shown that the relative error on kappa(S) scales as 1/root kappa(S) when kappa(S) is small. This implies that dynamical corrections to the TST rate constant can be computed efficiently if and only if the TST dividing surface has a transmission coefficient kappa(S) which is not too small. In particular, the TST dividing surface must be optimized upon (for otherwise kappa(S) is generally very small), but this may not be sufficient to make the procedure numerically efficient (because the optimal dividing surface has maximum kappa(S), but this coefficient may still be very small). (c) 2005 American Institute of Physics.