# [DFTB-Plus-User] DOS Fermi Energy problem

Fri Aug 21 08:47:38 CEST 2009

```Dear Ezgi,

> I would like to ask you the density of states (DOS) graphics. I have trouble
> around fermi level.
>
> Now here is my question is coming. In this geometry there should be confused
> Fermi Energy Level or I am calculating in wrong way? So which Fermi Energy
> Level in this DOS graphics should be done. From detailed.out file we can
> say:  Fermi energy: -0.1837716172 H       -5.000680 eV. To shift +Efermi
> Level or -Efermi Level or just draw this plot between -4  to 4 or what could
> be other solution? For my further calculation I have to get proper Fermi
> Energy Level and DOS graphs.
>

The Fermi level, which is spit out by the code is the absolute value of
the fermi level. So, if you want to generate a plot, where the Fermi
level is at zero, you should shift you plot by -Efermi, so in you case,
you'd had to add 5.00680 eV to every value. However, you should be aware
of the fact, that if you have a large gap and zero temperature, the
Fermi energy can be numerically quite unreliable, beeing just somewhere
in the gap. It is, however, not the case with your system.

Addiotionally, in order to draw a proper DOS, you should use more
K-points to have a better sampling, so I'd suggest to take a higher
K-point sampling. And of course, you have to write the number of
k-points generated by dftb+ for the given scheme into the input file for
dosplot.

Also I noted, that in your geometry, you have vacuum between the
sheets also along the periodic direction x. I'm not sure, if it was
intended or not.

Best regards

Bálint

--
Bremen Center for Computational Materials Science, University of Bremen