This was restricted to setting a number of torsion parameters to zero (that is, λ = ∞) and reducing the number of scans used in the fitting process. In particular, the aspartic acid ψ/φ distribution was improved by setting the χ 2 torsional parameters to zero. Additionally, using only the QM energy scan with the lowest minimum energy in the fitting process was shown to result in an improvement in the MD simulations of the dipeptides for the χ 1 torsional parameters of cysteine, methionine, serine and threonine. The need for manual input in the fitting process was also required for developing OPLS-AA/M and is likely due to the restrictive functional form of the torsional potential and the conformational dependence of the energy scans. The full set of manual changes involved is listed, along with the final torsional parameters, in Section S3.3.