Chelpg

CHELPG (CHarges from Electrostatic Potentials using a Grid based method)^[1] is an atomic charge calculation scheme due to Breneman and Wiberg, in which atomic charges are fitted to reproduce the molecular electrostatic potential (MESP) at a number of points around the molecule. ^{[2] [3]}

Interestingly the charge calculation methods based on fitting of molecular electrostatic potential (MESP) (including CHELPG) are not well-suitable for the treatment of larger systems, where some of the innermost atoms are located far away from the points at which the MESP is computed. In such a situation, variations of the innermost atomic charges will not lead to significant changes of the MESP outside of the molecule and fitting of these atomic charges will therefore not result in fuzzy results. It should be remembered that atomic charges depends strongly on the molecular conformation. The representative atomic charges for flexible molecules hence should be computed as average values over several molecular conformations.

CHELPG charges can be computed using the popular ab initio quantum chemical package Gaussian.

References

1. ^ Breneman, Curt M.; Wiberg, Kenneth B. (1990). "Determining atom-centered monopoles from molecular electrostatic potentials. The need for high sampling density in formamide conformational analysis". Journal of Computational Chemistry 11 (3): 361. doi:10.1002/jcc.540110311. 
2. ^ Jensen, Frank. Introduction to Computational Chemistry (2nd ed.). Wiley. ISBN 978-0-470-01187-4. http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470011874.html. 
3. ^ Cramer, Christopher J.. Essentials of Computational Chemistry: Theories and Models (2nd ed.). Wiley. ISBN 978-0-470-09182-1. http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470091827.html.