In this work, we can now attribute this result to basis-set dependence of the calculations, both on the computed geometry of the transition state and the resulting vibrational analysis on which the kinetic isotope effects are based. Other factors such as the continuum solvation model or the selection of a density functional method were less sensitive. We note that inclusion of an empirical term to correct for dispersion attractions did have a noticeable effect on the computed KIE values, as did the inclusion of an explicit solvent model for solvents where a hydrogen bond between the solvent and the substrate becomes possible. Our best suggested procedure, maximising accuracy whilst reducing computation times to a practical level, is at the B3LYP/Def-QZVPP level and corrected for third generation dispersion attractions and for a continuum solvation model.