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The two-dimensional band structure of a single epitaxial ErSi2 layer on Si(111)is calculated by means of the crystalline extension of the extended Hiickel method for various atomic structures and tested against experimental bands determined by angle-resolved photoemission. In particular, adopting for the silicide layer the structure proposed in previous work, i.e. , a hexagonal Er monolayer underneath a buckled Si top layer, various possible interfacial geometries are investigated, namely with the Er in top, substitutional, T4, and H3 sites of the Si(111)substrate and for the two possible orientations of the latter with respect to the buckled Si top layer. With the exception of the substitutional site, all models show two characteristic bands near the Fermi level that are essentially full and empty, respectively, as observed experimentally.Yet, the topology of these bands is correctly reproduced for only two interfacial geometries, namely Er in H3 (T4) sites with the buckled Si top layer having an orientation identical (opposite) to the substrate Si double layers. For both models the overall agreement between calculated and experimental bands is quite satisfactory. The prominent almost-filled band observed experimentally in the 0 -1.7-eV binding-energy range mainly derives from the dangling bonds of the buckled Si top layer, but shows a strong hybridization with Er 5d states near the center of the surface Brillouin zone.