Necessary conditions for the calculation of useful electrostatic isopotential maps from CNDO/INDO wavefunctions

The calculation of reliable electrostatic isopotential maps from CNDO or INDO wavefunctions while preserving the ZDO approximation fails because the individual atoms are treated as if they were spherical charge distributions. The resulting potential maps could be adequately interpreted on the basis of classical electrostatics. Approaches to the determination of useful maps include (a) the use of localized orbitals with a ZDO approximation applied among the localized orbitals as long as the localization procedure is defined with respect to the connectivity of the molecule and has no reference to a set of laboratory coordinates or (b) relaxation of the ZDO approximation in the determination of the potential by including the 3-center potential integrals. In either case, the preservation of rotational invariance takes paramount importance.