Approximation of Mulliken charges for the silicon atoms of all-siliceous zeolites

Distributed multipole analysis on the basis of periodic Hartree-Fock (PHF) calculations, using the CRYSTAL code, is applied to 13 all-siliceous zeolite with a reasonable number of atoms per unit cell (UC). Mulliken charges of the silicons were calculated with three basis sets: STO-3G for all frameworks, a 6-21G quality basis for five of them, and a 6-21G* quality basis set for chabazite. A simple one-dimensional function of the dependence of the charges obtained with the two first bases is found with respect to the average Si-O distance within the respective SiO₄ tetrahedra. Its application for the evaluation of the charges in frameworks with a larger number of atoms per UC is confirmed by comparison with results of direct PHF calculations for silicalite.