[Pw_forum] Energy window in STM simulation

[David Pullman]

Hello,
Sorry if you're getting this message twice, but I'm not sure the 
first one went through because our department is still in the process 
of changing email servers.

I have a question about QE's implementation of the the Tersoff-Hamann 
formalism for simulating STM images.  If I understand the stm.f90 
code correctly, the energy sampling window does not range from Ef to 
Ef+sample_bias (which is what Tersoff-Hamann says it should 
be).  Rather, the code increases the upper limit by 3*degauss 
(degauss=smearing width) and also decreases the lower limit by 
3*degauss.  In the case of metals, the value of degauss is taken from 
the prior PW run.  I believe the subsequent lines of code modify the 
weights of the states that are outside the Tersoff-Hamann window.

So, as an example, if a metal has a bias of -0.1 eV and the smearing 
width from the prior PW run was 0.01 Ry (or 0.136 eV), then states 
from -0.5 eV to +0.4 eV (with respect to Ef) are included in 
calculating the LDOS.  This strikes me as a rather broad range, even 
if temperature and energy linewidths are considered, and could alter 
the appearance of the computed images.

Why do the STM energy limits take into account the smearing width 
from the PW output?  And is it best to use as small a width as 
possible if you intend to run STM simulations?

Thanks,

David Pullman
Department of Chemistry and Biochemistry
San Diego State University
San Diego, CA  92182-1030