[Pw_forum] factional co-ordinate input to QE

Giuseppe Mattioli giuseppe.mattioli at ism.cnr.it
Wed Apr 8 11:12:57 CEST 2015

Dear Janardhan L.
It is not easy to answer your question in a simple way. Fractional coordinates generally indicates that multiple, often symmetric sites can be 
occupied by a number of atoms lesser than the number of sites. Moreover, H atoms are "reconstructed" by XRD fitting programs, because they are not 
"seen" as scatterers by X rays. Practically you can try to: a) find the most stable unit cell wrt the correct stoichiometric distributions of protons, 
and hope that it acceptably represents the properties of your system; or b) build a supercell and represent different populations of the sites in order 
to find low-energy configurations of protons. However, it is not at all easy to be sure that you are correctly representing your system in both the 
cases, so you should rely on some properties of the system which are "independent" from your choice, in order to find configurations which reproduce 
better observables related to such properties.

On Monday, April 06, 2015 02:40:50 PM janardhan H.L. wrote:
> Dear all,
> Giving input to QE as a slab is relatively easy and for bulk material it is usually done by converting cif files to readable xyz formats, cif files
> give only the irreducible coordinates which are obtained by fitting the experimental data. My question is on the second method, in many of the
> cases atomic positions are of fractional occupancy if we reproduce the complete unit cell by applying the symmetric operations it may lead
> different atomic co-ordinates. Example of zinchydroxystannate is attached, in this case both the hydrogen have 0.5 occ, by converting to xyz format
> by VESTA it gave a unit cell where the oxygen have 2 hydrogen but it is not true as it is the 0.5 occ for both hydrogen, in this case if I give occ
> of 1 for one of the hydrogen it is not the representative structure of the compound. So what is the best method to be followed to give a correct
> input of the following structure. with thanks and regards
> Janardhan L. HodalaClean energy catalysis labKIST,  S. Korea

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