CHELPG is an
atomic charge calculation scheme due to Breneman and Wiberg, in which atomic charges

Partial charge

A partial charge is a charge with an absolute value of less than one elementary charge unit .-Partial atomic charges:...

 are fitted to reproduce the molecular electrostatic potential

Electric potential

In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...

 (MESP) at a number of points around the molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

.
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

Conformational isomerism

In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...

. 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

Ab initio

ab initio is a Latin term used in English, meaning from the beginning.ab initio may also refer to:* Ab Initio , a leading ETL Tool Software Company in the field of Data Warehousing.* ab initio quantum chemistry methods...

 quantum chemical package Gaussian.