To compute the static EPs used in the fitting procedure, we first performed single-point energy calculations on the optimized structures using the Gaussian09 program. For this, the PBE0 exchange-correlation functional was used together with the aug-cc-pVDZ basis set for heavy atoms and the cc-pVDZ basis set for hydrogen atoms. In the following, we will denote this mixed basis set as augx-cc-pVDZ. These calculations formed the basis for the static EP evaluations of the capped amino acids. A similar set of calculations were carried out with a finite electric field to obtain induced EPs. Electric fields were applied along the ± {x, y, z} directions with a field strength of 5.0 × 10 -3 au, resulting in a total of seven different calculations for each structure (six applied fields and one field-free structure). The EPs were subsequently evaluated on a set of discretized molecular surfaces, using Horton 36 as a backend. The surfaces were generated according to the algorithm of Saff and Kuijlaars, with the radii of the spheres scaled according to the van der Waals (vdW) radii of the atoms. We used two shells in each structure, with scaling factors of 1.4 and 2.0. The construction of the surfaces, as well as the fitting of charges and polarizabilities, has been implemented in the PropertyFit Python package, freely available at . The charges and polarizabilities are obtained by minimizing the cost functions (eqs. ( ) and ( )) using the sequential least squares programming (SLSQP) optimization algorithm available in SciPy. The reference charges and polarizabilities used for the restraints (see eqs. ( ) and ( )) were obtained using the LoProp approach 8 based on the most probable structure of each amino acid, employing the PBE0 functional and the augx-cc-pVDZ basis set which has been recontracted to ANO-type as required by the LoProp procedure. The reference parameters were computed using the Dalton program 41,42 and the LoProp for Dalton Python package. The restraint strengths were set to k q restraint = 1.0 × 10 -6 au and k α restraint = 1.0 × 10 -9 au, chosen to be strong enough to avoid large drift of the parameters, but small enough that the quality of the fit did not deteriorate significantly. Reference charges and polarizabilities belonging to chemically equivalent atoms were averaged before use.