Fitting against the training set led to substantial improvements in the accuracy of the vibrational frequencies in the training data, relative to the reference QM results, as evident from a dramatic fall in the vibrational components of the objective function. This is evident in Figure , which shows decreases in error for the sum of squared vibrational frequency differences for individual molecules. Indeed, the improvement from initial results (red) to fitted results (green) appears even more marked than that of optimized geometries (Figure ). The distribution of improvements for individual conformations (blue) also shows strong improvement, with only a tiny minority of cases becoming less accurate with fitting. These results correspond to a 67% drop in the RMSE of individual MM vs QM frequencies; i.e., from 119 to 40 cm -1 (Table ). When the MM vibrational frequencies are permuted to maximize overlap between MM and QM normal modes, the RMSE in the vibrational frequencies is found to decrease by 42% (156 to 90 cm -1 ) with optimization.