Rumpled relaxation and surface core-level shift in PbS(001)

The surface relaxation of the PbS (001) surface has been calculated by the all-electron full potential linear augmented-plane-wave method. Surface core level shifts have been estimated at different atomic sites of the rumpled surface of the crystal. The rumpling is such that the cations are slightly below the anions in the surface layer. Comparison with recent experimental measurements by x-ray photoemission spectroscopy shows good agreement. The initial state approximation slightly overestimates the value of the surface core level shifts (SCLS) whereas the final state effect reduces the SCLS. It is shown that the Madelung potential cannot explain the shifts.