  The Pb/Si(111) thin films were simulated within the density functional theory
(DFT). The well-known Perdew-Burke-Ernzerhof (PBE) version of the generalized
gradient approximation (GGA) and its recent nonempirical successor Wu-Cohen
(WC) issue were used to estimate the exchange-correlation functional. Lattice
parameters were calculated for Bulk of the Pb and Si compounds to obtain more
reliable lattice mismatch at the interface to be consistent with our used
full-potential method of calculations. The WC-GGA result predicts the lattice
constants of the Pb and Si compounds better than the GGA when compared with
experiment. We have found that the spin-orbit coupling (SOC) does not
significantly influence the results. Our finding is in agreement with the
recent observation of the Rashba-type spin-orbit splitting of quantum well
states in ultrathin Pb/Si(111) films. Our result shows, in agreement with
experiment, that the top site (T1) is the most stable phase. A combination of
tight $\sigma$ and feeble $\pi$ bonds has been found at the interface, which
results in a covalent Pb-Si bond. Our calculated electric field gradient (EFG)
predicts quantum size effects (QSE) with respect to the number of deposited Pb
layers on the Si substrate. The QSE prediction shows that the EFG dramatically
drops on going from first to second layer. The EFG calculation shows that this
system is not an ideal paradigm to freestanding films.
