With scanning tunneling microscopy in air a large corrugation was obse
rved for the PbS(001) surface when using a small positive tip bias vol
tage. The observed periodicity of this corrugation does not correspond
to the crystallographic periodicity, but rather to the periodicity of
the wave function at a maximum of the valence band. This effect can b
e explained in terms of a charge density wave distortion at the surfac
e of the crystal.