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 simplified in the sense that one type of hydrogen atom is used to describe positively as well as negatively charged hydrogen atoms instead of two as in the original formula. The parameters are calibrated to sets of Mulliken charges obtained from STO-3G and STO-3G* calculations, containing 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 parameters (Ge and Ti excluded) are also calibrated to a set of potential derived charges (Merz-Kollman-Singh scheme). It seems that the EEM formalism, after appropriate parameterization, can reproduce the results of different charge partitioning schemes applied to calculations with different basis sets. Extending the EEM formula leads to a better reproduction of the charges. However, the parameters are highly correlated and, therefore, depend strongly on the calibration set used. All charges are 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 dynamics calculation on ethene absorbed in H-ZSM-5. It appears that the EEM approach in such calculations can help to understand chemical reactivity in zeolites. (C) 1998 Elsevier Science B.V. All rights reserved.