Over the years, fitting of the bonded parameters has perhaps received the most attention, due to their importance in determining the internal structure of molecules and proteins, and the relative ease of generating QM data for the molecules of interest. Fitting the atomic partial charges used in the Coulombic potential has also received significant attention, but is slightly more difficult, as mapping an continuous electrostatic potential onto a set of discrete atoms is conformation-dependent and involves a loss of fidelity. However, modelers have achieved good results using QM-based methods such as RESP and semi-empirical methods such as AM1-BCC .