The Electronegativity Equalization Method I: Parametrization and Validation for Atomic Charge Calculations

The applicability of the electronegativity equalization method (EEM) is investigated for the fast calculation of atomic charges in organic chemistry, with an emphasis on medicinal chemistry. A large training set of molecules was composed, comprising H, C, N, O, and F, covering a wide range of medicinal chemistry. Geometries and atomic charges are calculated at the B3LYP/6-31G* level, and from the calculated charges, effective electronegativity and hardness values are calibrated in a weighted least-squares fashion. The optimized parameter set is compared to other theoretical as well as experimental values and origins of the differences discussed. An approach toward extension of EEM to include new atoms is introduced. The quality of the EEM charges is assessed by comparison with B3LYP/6-31G* charges calculated for a set of medicinal molecules, not contained in the training set. The EEM approach is found to be a very powerful way to obtain ab initio quality charges without the computational cost of the ab initio approach.