  We introduce an orbital dependent electron tunneling model and implement it
within the atom superposition approach for simulating scanning tunneling
microscopy (STM) and spectroscopy (STS). Applying our method, we analyze the
convergence and the orbital contributions to the tunneling current and the
corrugation of constant current STM images above the W(110) surface. In
accordance with a previous study [Heinze et al., Phys. Rev. B 58, 16432
(1998)], we find atomic contrast reversal depending on the bias voltage.
Additionally, we analyze this effect depending on the tip-sample distance using
different tip models, and find two qualitatively different behaviors based on
the tip orbital composition. As an explanation, we highlight the role of the
real space shape of the orbitals involved in the tunneling. STM images
calculated by our model agree well with Tersoff-Hamann and Bardeen results. The
computational efficiency of our model is remarkable as the k-point samplings of
the surface and tip Brillouin zones do not affect the computation time, in
contrast to the Bardeen method.
