Periodic Hartree-Fock study of siliceous mordenite

The authors report results of an all-electron ab initio periodic Hartree-Fock LCAO calculation on a completely dealuminated form of mordenite, Si[sub 48]O[sub 96], possessing Cmcm symmetry. The reported physical properties were computed using a modified 6-21G* basis. The resulting calculation is one of the largest ab initio studies carried out to date and demonstrates the feasibility of using ab initio methods to investigate large, complex systems. The sensitivity of the total crystal energy to small changes in geometry is explored by comparing the relative stabilities of seven reported experimental mordenite structures. The authors note that the larger the standard deviation in Si-O bond distances and O-Si-O angles the less energetically favorable the lattice will be. Crystal charge density and deformation density maps have been computed in the cylindrical channel of one of the reported crystal structures. Those calculations indicate the absence of charge density in the center of the channel ([approximately] 10[sup [minus]6] e/bohr[sup 3]) and a polarization of charge density localized near the oxygen atoms. Ab initio electrostatic potentials and electric fields in chemically important regions of the material have been computed from the Hartree-Fock ground state density. Comparisons of the ab initio electrostatic potential to those resulting frommore » model charge distributions (Mulliken point charges and formal point charges) are made. The point charge models investigated did not provide a quantitatively correct description of the electrostatics of these lattices. The average Mulliken charges are +2.09[vert bar]e[vert bar] and [minus]1.04[vert bar]e[vert bar] for silicon and oxygen, respectively. The total valence density of states and projected density of states condensed to all silicon and all oxygen atoms as well as projections into oxygen 2s and 2p orbitals have been evaluated. 47 refs., 8 figs., 1 tab.« less