Although models I-III appear to be approaching converged values for both ∆ ‡ E and ∆ rxn E, using either harmonic or fixed-atom restraints, results for model IV are rather different when fixed-atom constraints are used. In our calculations, the ZPE-corrected barrier height increases from 8 kcal/mol (model III) to 14 kcal/mol (model IV), such that ∆ ‡ E(IV) is closer to ∆ ‡ E(I), after having decreased in the order ∆ ‡ E(I) > ∆ ‡ E(II) > ∆ ‡ E(III). (This is consistent with a sizable jump in the energetics between model III and model IV.2 that is reported in Ref. 50, from 9 kcal/mol to 13 kcal/mol under vacuum boundary conditions.) Although the same trend is observed when harmonic confining potentials are used, the magnitude of the effect is significantly suppressed. Similar trends are observed for ∆ rxn E. Results for model II thus appear to be converged to within < 2 kcal/mol of those for model IV, when harmonic confining potentials are employed. The same cannot be said when fixed-atom constraints are used.