Another very useful method for modeling molecule-molecule interactions are derived from a least squares fit to the electrostatic potential (ESP). ESP is one of the useful properties to acquire partial atomic charges suitable for modeling short-and long-range molecule-molecule interactions . The grid-oriented CHELP (Charges from Electrostatic potentials method) was the first method of its kind to be developed, being modified to CHELPG (Charges from Electrostatic Potentials using a Grid based method) by Breneman and Wiberg. The CHELPG method is less dependent upon molecular orientation than the original CHELP method in which partial atomic charges are fitted to reproduce the molecular ESP at a number of points around the molecule .