Variational transition state theory VTST rate constants

Variational transition-state theory (VTST), as its name implies, variationally moves the reference position along the MEP that is employed for the computation of the activated complex free energy, either backwards or forwards from the TS sttuctme, until the rate constant is minimized. Notationally... 

In Fig. 5.1, thermal rate constants obtained from accurate full-dimensional calculations are compared to transition state theory results [40] (left panel) and reduced dimensionality quantum calculations (right panel). Experimental results [41] are also displayed. Classical transition state theory (TST) and variational transition state theory (VTST) drastically underestimate the thermal rate constant. This shows that tunneling effects are very prominent in the H2 + OH reaction. Including different types of tunneling corrections [40], increased values of the thermal rate constant can be obtained. However, it should be noted that different tunneling corrections result in considerably different results. This reflects the ambiguity of quantum transition state... 

UFF (universal force field) a molecular mechanics force field unrestricted (spin unrestricted) calculation in which particles of different spins are described by different spatial functions VTST (variational transition state theory) method for predicting rate constants... 

Variational transition-state theory has been formulated on various levels [5, 23-27]. At first, there is the group of canonical VTST (CVTST) treatments, which correspond to the search for a maximum of the free energy AG(r) along the reaction path r [23, 24]. It was noticed early that for barri-erless potentials this approach leads to an overestimate of the rate constant because, in the language of SACM, channels are included that are closed. Therefore, an improved version (ICVTST) was proposed [25] that truncates Q at the position r of the minimum of (t(r) by including only states... 

A viable alternative for small systems is variational transition state theory or VTST (see Truhlar et al. 1985). Recall that TST makes use of the non-recrossing rule assumption. When recrossing does occur, the assumption results in the over-counting of transitions from reactants to products that is, the TST rate constant is an upper bound. In VTST, a divide is sought that minimizes these transitions resulting in a minimum rate constant and this divide becomes the basis for the VTST rate constant. We consider, as the simplest example, canonical variational ensemble transition state theory (CVT). 

The justification of variational transition state theory is rigorous only in a classical mechanical world because, when the local equilibrium assumption is valid, VTST provides an upper bound on the classical mechanical rate constant. One optimizes the definition of the transition state to minimize recrossing, and the calculated rate constant converges to the exact rate constant from above. 

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