where the sum runs over all dihedrals (k) in the molecule and V k 1-4 are parameters to be fit. We focus on reparameterizing the backbone (φ, φ , ψ and ψ ) and sidechain (χ 1 , χ 1 , χ 2 and χ 2 ) torsional parameters (Figure ). Re-fitting of the remaining torsional and improper parameters are beyond the scope of the current work, and these parameters are instead taken from the OPLS-AA/M force field. However, parallel efforts are being made to develop a toolkit for automated parameterization of small molecules using the QUBE force field, which will facilitate derivation of the remaining parameters in future. When fitting torsional parameters, the main objective is to minimize the difference between MM and QM gas phase dihedral energy scans. However, weighting schemes and regularization can also be used to change the form of the error function that is minimized.