An ob inilio €Iame.Fock perturbedcluster embedding scheme is applied to the problem of neutral defects in bulk LiF (specifically to Na substitutionals and the bound Schottky pair), using an extended basis set. In the present work we highlight the capabilitites and drawbacks of the method, in particular regarding the convergence of the procedure and the demiption of polarization and relaxation effects. We also compare our results with simulations based on the Mott-Littleton theory. Our results indicate that even in the case of neutral defects, there are important short-range polarization effects.

Silica sodalite is an ideal model system to establish base-line computer requirements of ab initio periodic Hartree-Fock (PHF) calculations of zeolites. In this article, we investigate the effect of various basis sets on the structural and electronic properties of bulk silica sodalite. We also study the interaction of He, Ne, and Ar with the sodalite cage. Our work shows that basis-set superposition errors (BSSE) in calculations using STO-3G and 6-21G(*) basis sets are as large as the interaction energies, leading to poor confidence in the results. To cure this problem, we present high-quality basis sets for Si, 0, He, Ne, and Ar, optimized for use with PHF methods, and demonstrate that the new basis set greatly reduces BSSE. The theoretical barriers for transfer of the rare gases between sodalite cages are 5.6, 13.2, and 62.1 kcal/mol for He, Ne, and Ar. 0 1996 John Wiley & Sons, Inc. ular species through the channels play an important role in zeolite chemistry [5,6]. Understanding the atomic level forces involved in adsorbatezeolite interactions is a necessary step in elucidating transport processes.This work focuses on the computational difficulties encountered in describing the nature of zeolite-adsorbate interactions using ab initio periodic Hartree-Fock (PHF) theory. me degree of interaction bemeen different adsorbates and fie host lattice varies dramatically. For example, neu-Z composed of corner-sharing SiO, and A10, tetrahedra. The channel structure resulting from the interconnected tetrahedral units allows zeolites to function effectively both as molecular sieves and catalysts [l-41.

Abstract. e-quartz and stishovite have been studied using a periodic ab-initio Hartree-Fock method in order to characterize the chemical nature of the Si-O bond and the way in which it changes with the coordination number of Si. Structural properties, including unit cell volume and c/a ratio have been optimized in order to evaluate the reliability of the method and the effect of the basis set. Density of states and electron charge density maps have been taken into account to investigate the electronic properties of the two systems and the r61e played by different orbitals. We also present comparisons with experimental X-ray emission data. The importance of d orbitals is stressed by our calculations, and a possible interpretation of the Si-O bond proposed. Quartz is found to be more covalent than stishovite.

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