Dear Wien2k users; for a tetragonal material, the birefringence is given by : delta_n=n_xx-n_zz or delta_n=n_zz-n_xx -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20181205/0f8f4fa2/attachment.html>