CHELPG

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CHELPG: A simple way to assign atomic charges

What CHELPG stands for
CHELPG means Charges from Electrostatic Potentials using a Grid-based method. It’s a way to assign partial charges to atoms in a molecule so the calculated electrostatic field around the molecule matches the real electrostatic potential (MESP).

How it works
- The method fits atom-centered charges so the MESP at many points around the molecule is reproduced as closely as possible.
- These charges are typically computed using common quantum chemistry packages and fit to a grid of points around the molecule.

When it’s best used
- CHELPG is designed for non-periodic systems, like isolated molecules, not bulk solids.

Limitations and alternatives
- For very large systems, inner atoms can be poorly determined by MESP-fitting, because their charges don’t strongly affect the outside potential. In such cases, density-based methods (like DDEC) partition the electron density to give charges that work well for both molecules and materials, including periodic systems.
- Because atomic charges depend on how the molecule is arranged (its conformation), it’s common to average CHELPG charges over several representative shapes for flexible molecules.

Other related approaches
- There are other MESP-based schemes that try to improve rotational invariance by using different point-selection methods, such as Connolly surface approaches or geodesic sampling.

What you can use it for
- CHELPG charges are widely used in computational chemistry workflows and can be obtained from programs like Gaussian, GAMESS-US, and ORCA.

Quick takeaway
CHELPG is a practical method to assign atom charges by fitting to the molecular electrostatic potential, best suited for small to medium non-periodic molecules, with alternatives like DDEC offering more robust options for large systems and periodic materials.