LARGE BASIS-SETS AND GEOMETRY OPTIMIZATIONS IN EMBEDDED-CLUSTER CALCULATIONS

In the embedded cluster method, a cluster is embedded in a correction potential. This potential adds the long-range electrostatic forces of the crystal and subtracts the electrostatic potential of the boundary errors of the cluster. The correction potential is calculated from the wave function of the cluster and the crystal. So far, the method has only been used on a cluster having the same geometry and basis set as that of the crystal. Here, we calculated the adsorption energies of NH 3 and NH4+ on an embedded zeolite cluster with different basis sets an d modified geometries. We showed that a mixed basis set, a basis set h aving a large basis set on the atoms around the adsorption site and a minimal basis set on the atoms of the boundary of the cluster, yields adsorption energies close to that of the large basis set. The optimize d geometries of the zeolite acidic site were almost equal for the none mbedded cluster, the embedded cluster, and the crystal. By combining t he mixed basis set with a partial geometry optimization, an improved d escription of the adsorption process is obtained. (C) 1995 John Wiley & Sons, Inc.