<div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr">Dear Wien2k users;<br><br><span class="gmail-tlid-translation gmail-translation"><span title="" class="gmail-">for a tetragonal material, the birefringence is given by :<br></span></span></div><div><span class="gmail-tlid-translation gmail-translation"><span title="" class="gmail-">delta_n=n_xx-n_zz <br>or </span></span><br><span class="gmail-tlid-translation gmail-translation"><span title="" class="gmail-"><span class="gmail-tlid-translation gmail-translation"><span title="" class="gmail-"><span class="gmail-tlid-translation gmail-translation"><span title="" class="gmail-">delta_n=</span></span>n_zz-n_xx<br></span></span></span></span></div></div></div></div>