For many biomolecular force fields, the parameters of the torsion potentials are obtained by fitting suitable periodic functions to energies evaluated at the MP2 level of theory. The parameters for each type of the torsional potential are simultaneously fitted for multiple amino acids. Therefore, the average root mean squared (RMS) difference between the torsional energy at the CHARMM36m level and the MP2 level of theory is of the order of 10 kJ mol -1 . The RMS deviation between the modified and original torsion potentials is at most 8 kJ mol -1 , and the RMS deviation between the ab initio potential at the MP2/6-31+G* level and the N-C α -C β -C γ torsion potential in ASP is reduced from 4 kJ mol -1 for the original CHARMM36m force field to 3.5 kJ mol -1 for modified CHARMM36m force field (Figure ). Therefore, we conclude that with the corrections of the torsion potentials, the modified force field provides an equally good fit to QM potential profiles as the original CHARMM36m force field. We also performed standard MD simulations and simulated five replicas for 100 ns for the two protonation states of the Asp tripeptide in water, using both the original and modified CHARMM36m force field parameters. Without the modifications, the local minima are not consistently sampled in all replicas (Figure ). In contrast, with the corrections, identical distributions of the dihedral angles are obtained also in standard MD simulations.