An embedding approach for the calculation of STM images: Method development and application to galena (PbS)

A point charge cluster embedding technique is developed to model the electronic structure of flat PbS surfaces. This method removes edge effects of the wavefunction that are most significant at the edges of the cluster, and it was applied to mimic a greater cluster thickness without a significant increase in computational cost. Point charges are fitted to compensate for the difference potential between the PbS cluster and an infinitely wide slab with a greater thickness than the cluster. STM images calculated with the embedding technique hardly show any edge effects. The differences of calculated STS spectra for tip positions above corner, edge, and center atoms of the cluster are much reduced after applying the point charges. Surface relaxation calculated for the bare cluster results in differences in the vertical relaxation between corner and center atoms at the surface of up to 0.5 Å. These differences decrease to less than 0.1 A with the embedding technique and the calculated relaxation agrees well with calculations using two-dimensional periodic slabs.