ON THE USE OF AM1 AND MNDO WAVE-FUNCTIONS TO COMPUTE ACCURATE ELECTROSTATIC CHARGES

A new strategy to evaluate accurate electrostatic charges from semiempirical wave functions is reported. The rigorous quantum mechanical molecular electrostatic potentials computed from both MNDO and AM1 wave functions are fitted to the point‐charge molecular electrostatic potential to obtain the electrostatic charges. The reliability of this strategy is tested by comparing the semiempirical electrostatic charges for 21 molecules with the semiempirical Mulliken charges and with the ab initio STO‐3G and 6‐31G* electrostatic charges. The ability of the dipoles derived from the semiempirical electrostatic and Mulliken charges as well as from the SCF charge distributions to reproduce the ab initio 6‐31G* electrostatic dipoles and the gas phase experimental values is determined. The statistical analysis clearly point out the goodness of the semiempirical electrostatic charges, specially when the MNDO method is used. The excellent relationships found between the MNDO and 6‐31G* electrostatic charges permit to define a scaling factor which allows to accurately reproduce the 6‐31G* electrostatic charge distribution as well as the experimental dipoles from the semiempirical electrostatic charges. Copyright © 1990 John Wiley & Sons, Inc.