APPROXIMATIONS OF THE MULLIKEN CHARGES FOR THE OXYGEN AND SILICON ATOMS OF ZEOLITE FRAMEWORKS CALCULATED WITH A PERIODIC HARTREE-FOCK SCHEME

Distributed multipole analysis (DMA) on the basis of periodic Hartree- Fock (PHF) calculations, using the CRYSTAL code, is applied to five di fferent all-siliceous zeolite models: chabazite, gmelinite, meplinoite , montesommaite, and RHO. Mulliken charges of the framework atoms were calculated with a pseudopotential ps-21G basis set for silicon and a 6-21G'' basis for oxygen. The charge values of the silicon atoms were approximated by a simple one-dimensional function with respect to the average Si-O distance within the respective SiO4 tetrahedra whereas a two-dimensional function with respect to the average Si-O distance an d the Si-O-Si angle was used for the oxygen atoms. Both dependences we re then utilized to evaluate the Mulliken atomic charges of 10 other f rameworks with a larger number of atoms per unit cell. The validity of such application is confirmed by comparison with results obtained thr ough direct PHF calculation for all-siliceous mordenite. (C) 1998 John Wiley & Sons, Inc.