Point-charge Description of some Molecular Properties*

The notion of formal atomic charges in molecules is probably the most debated issue in quantum chemistry. Although atomic charge can not be rigorously defined in a unique way, conclusive evidence is given here which shows that it provides semiquantitative information about a number of molecular properties in a very simple and transparent way. In particular, the calculations of ESCA chemical shifts, diamagnetic shielding of nuclei and diamagnetic part of the molecular magnetic susceptibility are well described and thoroughly discussed. Finally, a relation between the effective atomic charges and total molecular SCF energies is illustrated by numerical examples. The point-charge description of the mentioned molecular properties is particularly useful if it is employed within the framework of semiempirical theories .because the computational costs are then negligible. The most successful semiempirical scheme in this respect seems to be the SCC-MO (self-consistent charge MO) method.