A LOCALIZED ORBITALS BASED EMBEDDED-CLUSTER PROCEDURE FOR MODELING CHEMISORPTION ON LARGE FINITE CLUSTERS AND INFINITELY EXTENDED SURFACES

A new embedded cluster procedure for modeling chemisorption on metal s urfaces is developed. The procedure is similar in philosophy to the ap proach used by Whitten and co-workers in that energy calculations are performed in a cluster region basis consisting of localized occupied a nd virtual orbitals. However, we present a new localization procedure to generate the cluster region functions which is based on orbital occ upation numbers determined from the density matrix obtained in a calcu lation on the extended substrate. Our localization procedure avoids ha ving to perform separate unitary transformations on the canonical occu pied and virtual orbitals and as a consequence has the attractive feat ure of enabling the embedded cluster calculations to be applied to bot h large finite clusters and infinitely extended systems in essentially the same manner. We illustrate the embedded cluster procedure by perf orming partial SCF calculations in the cluster region basis for H adso rption at an on-top site of a Li(100) monolayer. When the extended sur face is modeled by large finite clusters, the localized orbitals in th e cluster region rapidly converge to being completely occupied or comp letely empty, and we find partial SCF calculations to readily reproduc e the full SCF results of the large finite cluster. For the infinitely extended surface, the occupation numbers for the localized functions in cluster regions converge much more slowly than in the finite case, but even with less than perfect occupation numbers we still obtain goo d H adsorption properties in the partial SCF calculations. Unlike the finite cluster case where charges are automatically balanced, we found in order to achieve good results in the partial SCF calculations on t he infinitely extended systems it was necessary to carefully balance t he charges used in the long range electron and nuclear interactions. A ll of the calculations involving clusters are performed with the GAMES S program and the calculations on the infinite extended surface are pe rformed with the periodic Hartree-Fock CRYSTAL program. (C) 1996 Ameri can Institute of Physics.