[Pw_forum] Site Occupancy

[Lazaro Calderin]

Hi Ahmet,

> 
> I am trying to obtain the band structure of Ammonium
> Hexafluorosilicate-(NH_4)_2SiF_6-where the space group is Fm3m,
> location and occupancies* of the base atoms are:
> 
> atom           position                        occupancy
> ------------------------------------------------------------------------
> Si               (0,0,0)                             1
> F                (0.2011,0,0)                     1
> N                (0.25,0.25,0.25)               1
> H                (0.168,0.168,0.269)          1/3
> 
> When the structure is contructed via symmetry operations, the number
> of the H atoms are three times the actual value. Could you please help
> me in how to use the knowledge of "occupancy" of the H atoms so that
> the band structure calculation gives results corresponding to the
> physical structure?

Occupancies are usually not a direct input for electronic structure
calculations. To simulate the occupancies one just reduces the number of
atom in the structure by hand, I mean deleting those atoms that are not
supposed to be there. Sometimes it is obvious what sites should be
occupied or empty, for example when H ions are very close to each other.
However if it is not the case then you should generate all the
non-equivalent possibilities of having 1/3 of the H positions empty and
fully relaxed them under the same computational conditions to find the
most stable configuration(s).

Lazaro

-- 
=============================================================
Lazaro Calderin                      
Department of Physics        
Queen's University, Kingston, Ontario, Canada 
E-mails: calderin at physics.queensu.ca