Limits of perturbation theory: first principles simulations of scanning tunneling microscopy scans on Fe(100)

Scanning tunneling microscopy (STM) scans on Fe(100) are compared with firs t principles calculations of the tunneling current based on the transfer Ha miltonian method. Experimentally, we find a reversal of corrugation for sep arations between sample and tip below 400 pm. In the simulations we can rep roduce these topographies only in a distance range above 400 pm. The approa ch therefore fails to describe the observed corrugation reversal. We sugges t that this failure is due to the quenching of surface states by the approa ching STM tip. (C) 2000 Elsevier Science B.V. All rights reserved.