Each of these densities are initially fit using a modified version of RESP electrostatic fitting using Connolly surfaces that is retrofitted for optimizing site-centered densities. In order to clarify the difference between the two fitting procedure, we will hereby refer to the density based version as Density RESP or DRESP. In this procedure, frozen densities are determined by minimizing a lagrangian L DRESP based upon the mean-square difference between a DFT computed electrostatic potential and the potential generated from the model densities,