This page describes how to generate, using Fireball, the tip_e_str.inp with the retarded and advanced green functions and the density of states (DOS) for the STM program . First, we need to obtain SCF-solution for particular geometry. Once we have it, we need to switch on these options:
ifixcharge = 1 iwrtdos = 1
Simulation parameters are defined in two additional files:
This file defines basic parameters of the DOS calculation.
1.00000 ! ratio of lattice (for fractional coordinates, if xyz are in Angstroms maintain 1.000) 1 5 ! natom_beg,natom_end (initial and final atom for the DOS calculation) 401 ! number of energies (number of energies in the DOS calcuation) -12.0 0.05 ! first energy (initial absolute energy (fireball's value is the Fermi Level) and energy step 1 ! 1/0 yes/no write the tip_e_str.inp file for the DOS -4.01 4.01 ! minimun and maximum energy limit for writing the green function in the tip_e_str.inp file 0.05 ! imaginary part for the green function in the DOS calculation
One comment related with the minimum and maximum energy limits. Normally, we are interested in the range 2eV bellow and above the Fermi Level, but calculating a bigger range of DOS. These parameters give us the opportunity to do both things together: dos calculation and tip_e_str.inp fie generation. As an example we can consider a system with a FL=-2.0 eV and we want to generate the file from -2+FL to FL+2 doing the calculation of the complete DOS from -10 to +10 with an energy step of 0.05. The corresponding file is showed above. (Take a look to the minimum and maximum vales a little bit larger to avoid numerical bad effects).