Accurate computation of chemical contrast in field ion microscopy

We present a computational approach to simulate local contrast observed in field ion microscopy (FIM). It is based on density-functional theory utilizing the Tersoff-Hamann approach as done in scanning tunneling microscopy (STM). A key requirement is the highly accurate computation of the surface states' wave-function tails. To refine the Kohn-Sham states from standard iterative global solvers we introduce and discuss the EXtrapolation of Tails via Reverse integration Algorithm (EXTRA). The decaying tails are obtained by reverse integration (from outside in) using a Numerov-like algorithm. The starting conditions are then iteratively adapted to match the values of plane-wave Kohn-Sham wave functions close to the surface. We demonstrate the performance of the proposed algorithm by analyzing and showing the chemical contrast for Ta, W, and Re at Ni surface.