We then released the G03 constraints and reoptimized this system using modern versions for the G16 keywords specifying higher accuracies using the keywords: scrf=(iefpcm,solvent=dichloromethane,read), scf(conver=10), integral=(ultrafinegrid,acc2e=14) and opt(calcfc,ts,noeigentest,cartesian). We obtained -823.543287 for the total energy (cf. -823.545644624 9 ), -823.396527 for DG298.15 (cf. -823.400723 9 ), 5.15e-06 for the final gradient norm and 287.03i cm -1 for the imaginary mode wavenumber. The latter is only a small deviation of ~1 cm -1 induced by the change in program parameters from G03 to G16 and which may be attributed to greater two-electron integral accuracy. The calculated gas phase value of the imaginary mode using G16 for endo-TS was 380.51i cm -1 , in accord with the original observation 9 that a polarizable continuum model reduces the imaginary frequencies by approximately 100 cm -1 . DG298.15 for endo-TS in dichloromethane emerges as +2.63 kcal/mol higher than that quoted in the ESI, a result probably due in part to a combination of the use of modern values for the fundamental constants, tighter convergence criteria and a more accurate grid for the two-electron integrals.