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

[sonu kumar]

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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