Editing ChargeCalculator:FAQ (section)

=Can I get good dipole moments?=

''Atomic 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. 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 are generally not well described using atomic charges.

Dipoles and higher order multipoles are known to be poorly approximated by a point charge model. Dipole moments measure the degree of separation of positive and negative charge in the molecule (polarity), and are, by definition, very sensitive to small variations in the distribution of electron density. Even atomic charges computed at the quantum mechanical (QM) level have trouble reproducing dipole moments, though some ''charge definitions'' (see above) are less unsuccessful than others (MK charges can be satisfactory for estimating dipole moments for small molecules). It is thus clear that one cannot expect the accuracy of empirical models fitted to QM reference charges to exceed the accuracy of QM atomic charges in reproducing dipole moments.

Nonetheless, it is not unlikely that you obtain reasonable results for relative dipole moments in series of derivatives of a certain chemical compound. In other words, the atomic charges obtained from '''ACC''' calculations may not provide accurate dipole moments for a single molecule, but it is possible to compare the polarities of many kinds of derivatives of this molecule. Note that the current implementation of '''ACC''' provides only the values of atomic charges, and you will have to compute the dipole moments yourself. In the future '''ACC''' might support such functionality.

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