[Pw_forum] further: born charges in rhombohedral unit cell.

[Stefano de Gironcoli]

I don't  know how to explain.. but when things are exactly zero it's 
because there is a symmetry reason.
like a mirror plane or something ...
If your symmetry allows the born effective charge to be non zero they 
will, much or little depending on the details of the system.

Are the symmetry operations determined in the two systems the same ?

stefano

On 04/08/2011 10:19 AM, sonu kumar wrote:
> Sorry for my incomplete mail.
>
> Dear Prof. Stefano de Gironcoli  and QE users,
>
> So one would expect to have anisotropic born charges with off-diagonal
> elements for unit cell with R-3m space group.
> However, in PHYSICAL REVIEW B 73, 155114 ͑2006͒ , NaHF2   have zero values
> for off-diagonal
> elements of born-charges and diagonal elements are:
>
>                H             F                   Na
> xx/yy   .337        ~ -.7            1.057
> zz         ~2     ~ -1.5         ~1
>
> Similary in CuAlO2 with same space group, i have found zero values of
> off-diagonal elements of born tensor.
>
> How you explain this?
>
> With kind regards,
> Sonu Kumar
> IITD
>
>
>
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