Modification of the local self-consistent field method for modeling surface reactivity of covalent solids

The local self-consistent field (LSCF) method which allows full SCF computations on a fragment of very long molecular systems represented by a classical force field has been adapted to the description of nonmetallic crystals. The periodicity of the network is achieved by modifying self-consistently the basic parameters of the classical subsystem (charges, geometric parameters) along the SCF iterative scheme. The method is tested on cr-cristobalite. The parametrization of the quantum classical junction, achieved by localized bond orbitals has been performed with the help of a fragment located in the bulk. The stability of the method with respect to the size of the fragment and the size of the crystalline sample appears to be very good. The properties of fully hydroxylated (010) surface are corrected described. Modeling of water adsorption on the ideal surface as well as on two kinds of surface defects gives rise to very reasonable results with an absorption energies of ca 10 kcal/mol which are close to the upper limit of the experimental data. This preliminary study appears to be quite encouraging regarding the possibilities of using this method, which can be considered as an extension of the embedded cluster approach to covalent solids. Many applications to surface chemical reactivity studies can be imagined. (C) 1996 John Wiley & Sons, Inc.