A new embedded cluster procedure for modeling chemisorption on metal surfaces is developed. The procedure is similar in philosophy to the approach used by Whitten and co-workers in that energy calculations are performed in a cluster region basis consisting of localized occupied and virtual orbitals. However, we present a new localization procedure to generate the cluster region functions which is based on orbital occupation numbers determined from the density matrix obtained in a calculation on the extended substrate. Our localization procedure avoids having to perform separate unitary transformations on the canonical occupied and virtual orbitals and as a consequence has the attractive feature of enabling the embedded cluster calculations to be applied to both large finite clusters and infinitely extended systems in essentially the same manner. We illustrate the embedded cluster procedure by performing partial SCF calculations in the cluster region basis for H adsorption at an on-top site of a Li(100) monolayer. When the extended surface is modeled by large finite clusters, the localized orbitals in the cluster region rapidly converge to being completely occupied or completely empty, and we find partial SCF calculations to readily reproduce 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 good 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 the infinitely extended systems it was necessary to carefully balance the charges used in the long range electron and nuclear interactions. All of the calculations involving clusters are performed with the GAMESS program and the calculations on the infinite extended surface are performed with the periodic Hartree-Fock CRYSTAL program.