appearing in kate at So, but one first determines at what geometry(s")the ts partition function (i.e, AB, or qa)is smallest. Because this partition function is a product of (i)the exp(-E(S)kT) factor as well as(1i)3 translational, 3 rotational, and 3N-7 vibrational partition functions(which depend on S), the value of s for which this product is smallest need not be the conventional TS value So. What this means is that the location(s")along the reaction path at which the free-energy reaches a saddle point is not the same the location So where the born-Oppenheimer electronic energy E(S)has its saddle. This interpretation of how S* and so differ can be appreciated by recalling that partition functions are related to the helmholtz free energy a by q exp(-A/kT); so determining the value of s where g reaches a minimum is equivalent to finding that s where a is at a maximum So, in VTSt, one adjusts the "dividing surface"(through the location of the reaction coordinate S)to first find that value S" where krate has a minimum. One then evaluates both e(s")and the other components of the Ts species partition functions at this value S*. Finally, one then uses the kate expressions given above, but with S taken at s". This is how vast computes reaction rates in a somewhat different manner than does the conventional tst. as with tst. the vtst in the form outlined above, does not treat tunneling and the fact that not all trajectories crossing proceed to products. These corrections still must be incorporated as an"add-on to this theory (i.e., in the K factor) to achieve high accuracy for reactions involving light species(recall from the Background Material that tunneling probabilities depend exponentially on the mass of the tunneling particle)9 appearing in krate at S0 , but one first determines at what geometry (S*) the TS partition function (i.e., qAB* or qA*) is smallest. Because this partition function is a product of (i) the exp(-E(S)/kT) factor as well as (ii) 3 translational, 3 rotational, and 3N-7 vibrational partition functions (which depend on S), the value of S for which this product is smallest need not be the conventional TS value S0 . What this means is that the location (S*) along the reaction path at which the free-energy reaches a saddle point is not the same the location S0 where the Born-Oppenheimer electronic energy E(S) has its saddle. This interpretation of how S* and S0 differ can be appreciated by recalling that partition functions are related to the Helmholtz free energy A by q = exp(-A/kT); so determining the value of S where q reaches a minimum is equivalent to finding that S where A is at a maximum. So, in VTST, one adjusts the “dividing surface” (through the location of the reaction coordinate S) to first find that value S* where krate has a minimum. One then evaluates both E(S*) and the other components of the TS species partition functions at this value S*. Finally, one then uses the krate expressions given above, but with S taken at S*. This is how VTST computes reaction rates in a somewhat different manner than does the conventional TST. As with TST, the VTST, in the form outlined above, does not treat tunneling and the fact that not all trajectories crossing S* proceed to products. These corrections still must be incorporated as an “add-on” to this theory (i.e., in the k factor) to achieve high accuracy for reactions involving light species (recall from the Background Material that tunneling probabilities depend exponentially on the mass of the tunneling particle)