We explored method dependency using only the endo-TS series (Table ) by evaluating different pairs of DFT functionals and basis sets and one semi-empirical method and using the more recent Gaussian 16 solvation models rather than replicating G03 parameters. The semi-empirical PM7 procedure ( ). At the B3LYP/Def2-QZVPP level, where the basis set may be assumed to be approaching the complete basis set (CBS) limit, results such as dichloromethane (KIE250 2.08± 0.07, calc 2.136) suggest the method is now slightly overshooting experiment, but this overshoot is reduced (KIE250 2.08± 0.07, calc 2.066) when a third-generation dispersion correction 47 is added to the method (Figure ). Another method which includes a second-generation dispersion correction (ωB97XD/Def2-QZVPP) clearly over-estimates the KIE250 (2.08± 0.07, calc 2.311). We also note that the results are relatively insensitive to the precise solvation model used (IEFPCM vs CPCM). Our final and computationally the most expensive procedure is the double-hybrid B2PLYP-D3 50 /Def2-QZVPP method which includes dispersion and correlation corrections at several levels. The transition state geometry 51 is similar to the previous methods (rS-O 2.264Å, rC-H 1.19/1.71Å, aCHC 140°) and results in the following values of KIE for endo: 2.215 @250K and 2.776 @195K and for exo: 2.276 @250K and 2.874 @195K. The calculated exo/endo ratios are respectively 2.47@195K and 2.22@250K. When the individual KIE are combined using eqn.