EXTENDING AND SIMPLIFYING THE ELECTRONEGATIVITY EQUALIZATION METHOD

The Electronegativity Equalization Method (EEM), developed by Mortier et al. [J.W. Mortier, S.K. Ghosh, S. Shankar, J. Am. Chem. Sec., 108 ( 1986) 4315; G.O.A. Janssens, B.G. Baekelandt, H. Toufar, W.J. Mortier, R.A. Schoonheydt, J. Phys. Chem., 99 (1995) 3251], is extended with a shielded external potential to improve its accuracy. EEM is also simp lified in the sense that one type of hydrogen atom is used to describe positively as well as negatively charged hydrogen atoms instead of tw o as in the original formula. The parameters are calibrated to sets of Mulliken charges obtained from STO-3G and STO-3G* calculations, conta ining Al, C, H, N, O and Si atoms, and also Ge and Ti atoms for which no parameters were found in literature yet. Furthermore, the parameter s (Ge and Ti excluded) are also calibrated to a set of potential deriv ed charges (Merz-Kollman-Singh scheme). It seems that the EEM formalis m, after appropriate parameterization, can reproduce the results of di fferent charge partitioning schemes applied to calculations with diffe rent basis sets. Extending the EEM formula leads to a better reproduct ion of the charges. However, the parameters are highly correlated and, therefore, depend strongly on the calibration set used. All charges a re well reproduced, except on titanium. A sensitivity analysis of the charges with the original and extended EEM formalism shows that their results differ, but are strongly correlated. The applicability of the EEM approach and the parameters derived is shown in a molecular dynami cs calculation on ethene absorbed in H-ZSM-5. It appears that the EEM approach in such calculations can help to understand chemical reactivi ty in zeolites. (C) 1998 Elsevier Science B.V. All rights reserved.