Editing ChargeCalculator:FAQ (section)

=What is the physical meaning of atomic charges?=

''Atomic charges'', or ''atomic partial charges'', are non-integer numbers quantifying the balance of positive (nuclear) charge and negative (electronic) charge associated with each atom. In the 3D space, atomic charges represent points placed at the position of the atomic nuclei, and may be termed ''atomic point charges''. The molecular representation based on atomic point charges is thus a very basic abstraction of the molecular electron density. 

Atomic charges are conceived to reflect the uneven distribution of electron density in the molecule. While atomic charges are merely concepts and not physical observables, they have been used heavily in theoretical and applied chemistry due to their highly intuitive character and correlation with measurable quantities such as the electrostatic potential, polarity, reactivity, etc. Nowadays, atomic charges are still integral parts of many modeling applications, and are still used in reasoning basic chemical processes. 

When employing atomic charges, you must be aware of the limitations inherent to the atomic point charge model. A single number can give an idea about whether there is more electron density around some atoms compared to others, but it cannot characterize the actual distribution of electron density in the space between the atomic nuclei. Thus, all properties which flow from this distribution (such as multipole moments) are generally not well described using atomic charges.