Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the atomic and electronic structure of the T1/Si(1)-(root 3x root 3) surface. In the (root 3x root 3) reconstruction of surface, the two possible adatom geometries were considered: adsorption in Mold symmetric hollow sites (H-3 model) and in 4-fold atop sites (T-4 model). We have found that the energy difference between these two models is indeed very small within the energy of 0.03 eV. By calculating the electronic band structure for the T4 configuration, we have also identified two occupied and one unoccupied surface state. We have also determined the origin of these surface states. Our results are seen to be in agreement with the recent angle-resolved photoelectron spectroscopy. (c) 2008 Elsevier B.V. All rights reserved.