The QM ESP (Φ ref i ) is calculated from the partitioned atomic electron density (n i (r)). This is advantageous as the method may be applied equally well to both surface and buried atoms. The ESP is taken at a series of points on sets of spheres with radii between 1.4 -2.0 times the van der Waals radius of the atom. The error F (Φ, Φ ref ) is given by: