As the non-bonded parameters used are specific to the system under study, they are not the same for an alanine dipeptide molecule as for the alanine pentapeptide (Ala 5 ). The alanine residue in the dipeptide is blocked by acetyl and N-methyl groups whereas the central three alanine residues in Ala 5 have neighboring alanine residues on both sides. Therefore, varying environments exist for alanine residues in the different molecules. Consequently, the parameters found for the alanine dipeptide were found to be unsuitable for MD simulations of Ala 5 (Table ). However, the use of a harmonic restraint in the fitting process resulted in torsional parameters that were sufficiently accurate for the alanine and glycine peptide simulations. Alanine and glycine backbone torsional parameters were refit to the QM energy scans with λ = 0.50, no weighting was used. The optimal value used for the regularization parameter was found by minimizing the differences between simulated and experimental NMR observables for Ala 5 (Table ). We note that the J coupling error is not too sensitive to the strength of the harmonic restraint. Separate torsional parameters are used for the alanine pentapeptide and glycine tripeptide (Gly 3 ), as residue-specific parameters should result in a more accurate force field.