[Pw_forum] why do we need to relax the atomic coordinates obtained from cif file

[Axel Kohlmeyer]

On Sun, Sep 21, 2014 at 1:44 PM, janardhan H.L. <janardhanhl at yahoo.com> wrote:
> Dear all,
>
> I am an experimentalist wanted to use QE for obtaining DOS and band
> structure. As I was recently got introduced to QE and also most of the
> theory, I have a small question. I searched in PW archives and didnt find
> the answer looking for. My question is as follows,
>
> While solving a crystal structure we take lots of parameter and refine the
> crystal structure to at most accuracy. If we take this atomic coordinates
> for QE scf calculation does it affect the results obtained?

yes.

> If yes why, and relaxing the atomic positions may result in different
> coordinates which may be different from the input file which may not give
> the similar XRD patterns. Hence by relaxing the atomic coordinates from cif
> file and using for further calculations may not match to experimental
> results. Please comment on this.

there are assumptions in *both*, your DFT calculations and your x-ray
diffraction pattern refinement. your "most accurate" results may not
that be as accurate on a absolute scale, they are for the most part
*consistent* with the model that you use to refine your crystal
structure. the same is true for DFT calculations and wanting a
geometry that is *consistent* with the model is usually desirable.
using the unoptimized coordinates may result in a wavefunction that is
"squeezed" into a situation that it doesn't really want to be and then
will "act funny".

in general, the differences in relative geometries are fairly small,
since in both cases a lot of error cancellation is happening. if you
cannot get a good agreement between the experimental results and the
DFT calculation, that is usually an indication that something is going
badly wrong (or that you may be on the brink of a nobel prize worthy
discovery)

HTH,
    axel.

>
>
>
> with respectful regards
>
> Janardhan H L
> Research Fellow
> Poornaprajna Institute of Scientific Research
> Bangalore India
> www.ppisr.res.in
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum



-- 
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.