Locating a transition state is best achieved using a calculated force constant matrix at the initial geometry, which as noted above requires numerical calculation for the G03 code. The modern keyword used in G16 for such location is opt(calcfc,ts), but this presumes that analytic force constants are available. We decided to see whether using only G03 options, the gradient norm could be reduced below 0.001477267 Hartree by adopting the following method. Using the original geometry with no optimization, the keyword freq(numer) was used to store the force constant matrix for this geometry in a checkpoint file. This was followed by a second otimization job providing this force constant file using the keyword opt(readfc,ts). Several cycles of this procedure eventually reduced the gradient norm to an acceptable value of 0.000010283 for dichloromethane as solvent. The final corresponding DG298.15 = -823.399837 Hartree 22 is higher than the reported value 9 by 0.55 kcal/mol, in part at least because reducing the gradient norm tends to increase the computed values of low energy normal modes, resulting in a smaller entropy and hence a higher free energy. At this newly re-optimized geometry, the transition state normal mode is reduced to 281.5i cm -1 and now compares much better with the literature value for dichloromethane of 286i cm -1 (Table ). There is something of a disconnect between the accurate replication we obtained for total energies using geometry coordinates quoted for a dichloromethane model in the article ESI9 and our still reasonable replication for the transition state mode wavenumber in the same solvent, albeit using our re-optimized coordinates, values for which do not appear to be present in the article ESI. 9