Work function and surface energy per unit area were calculated in the framework of density functional theory (DFT) with Linearized A ug mented Plane Wave Plus Local Orbital method in full potential for a clean symmetric slab of silicon containing two (top and bottom) surfaces. The surfaces were theoretically modeled using supercell technique by stacking a variety of silicon layers along (111) crystallographic direction. In order to make the slab symmetric , two equivalent amounts of vacuums were added on the top and bottom of the Si(111) surfaces. In order to simulate the interface between lead and silicon, thin films of Pb were deposited on the prepared Si(111) surfaces. The calculations were performed in the absence and presence of spin-orbit coupling (SO) for the three phases of top site, T1, and fcc, T4, as well as hcp, H3. Our calculated total energies in agreement with experimental measurements show that the T1 phase, for which Pb atom is located along the Si atom of the first silicon layer, is the most stable phase. For the interface of Pb/Si(111) work function and energy formation were then calculated obtaining the most stable top site phase into account. The Si-Pb bond in the interface has been determined to be sigma.