Modelling Electrostatic Potential from Experimentally Determined Charge Densities. II. Total Potential

Experimental electrostatic potential derived from X-ray diffraction data was used as a given physical property for the determination of atomic moments. The electrostatic potential is fitted against Buckingham moments expansion up to octupolar level. The estimation of the contribution of the aspherical part of the density to the electrostatic potential necessitates a judicious choice of the points grid around the system in order to get stability and reliability of the results. The net charges obtained by the fit to the electrostatic potential on a test crystal of the pseudo-peptide N-acetyl-α,β-dehydrophenylalanine methylamide are close to those derived from the electron-density refinements. The higher moments are related to the electron-density multipolar parameters.