APPROXIMATION OF THE MULLIKEN-TYPE CHARGES FOR THE OXYGEN-ATOMS OF ALL-SILICEOUS ZEOLITES

Distributed multipole analysis on the basis of periodic Hartree-Fock ( PHF) calculations, using the CRYSTAL code, is applied to 12 all-silice ous zeolite models, plus one H-form type. A simple approximation of th e dependence of the Mulliken-type charge of the framework oxygens, cal culated with two Gaussian basis sets, is found with respect to the ave rage Si-O distance and Si-O-Si angle. These results allow the estimati on of the oxygen charges within zeolites with larger elementary unit c ells which are still hardly tractable with the presently available com puting facilities. The validity of such an estimation for the oxygens of silicalite is shown by comparison with results of direct PHF calcul ations. (C) 1998 Elsevier Science B.V. All rights reserved.