Although this was anticipated, it is important to confirm, because it verifies the effectiveness of the optimization procedures and provides a scale for the degree of improvement. The dimensionless objective (or loss) function-the weighted sum of squared differences between QM and MM values-decreased dramatically in the fitting, from 25,708 to 4,522 (Figure ). As described in Section 2.1, the objective function is a sum of contributions which report the accuracy of optimized geometries, vibrational spectra, and torsional energy profiles. The effect of training on these components is summarized in Table (Training Set data)