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