THE ELECTROSTATIC POTENTIAL IN THE SEMIEMPIRICAL MOLECULAR-ORBITAL APPROXIMATION

A method is described for calculating the molecular electrostatic potential (MEP) within the MNDO, AM1 or PM3 semiempirical approximations. The MEPs are used to obtain atomic charges from which dipole moments are calculated for comparison with the exact semiempirical values. The results for a wide range of neutral molecules and ions show excellent agreement between the dipole moments calculated from MEP-derived atomic charges and the exact semiempirical dipole moments. The neutral molecules and cations showed a mean deviation in the dipole of less than 0.01 D. The maximum errors (almost-equal-to 0.04 D) were obtained for some molecules containing third row or heavier elements. By contrast, all previously reported methods based on the semiempirical approximations for generating MEP-charges in general yield significantly larger deviations from the exact dipoles. The maximum error for the anions tested was 0.07 D, with a mean deviation only slightly higher than for neutral species and cations. The correct limiting behaviour is obtained when the MEP surfaces are calculated at large distances, i.e. the exact dipole is approached asymptotically. Quadrupole moments of the MEP-derived charge distributions are reported for some nonpolar and polar molecules.