Transition state theory calculations for k∞(T) are similar to calculations of Keq(T). However, transition states more difficult to compute accurately than stable molecules: it is more difficult to provide guesses at transition state geometries that will successfully converge to the desired saddle point, and there are often low-lying electronic states which can cause convergence difficulties and degrade the accuracy of common approximations in the electronic structure calculations. At a simpler level, we do not have good empirical bond additivity corrections for transition states, so at best we might expect errors in barrier heights similar the green histograms in Fig. . The implication is that if we want |δE| < RT to keep our computed k's within about a factor of 3 of the true values, we need to use rather expensive quantum chemistry methods (at least CBS-QB3, maybe CCSD(T)-F12).