I have one query about data obtained in the optical calculation (absorption coefficient) using GGA. The output came in the format energy xx yy zz xy yz zx for bulk system. I have seen in the literature while showing for absorption coefficient, only two components E||a, and E||c have been shown. In my case, how I can decide which component is a or c. I potted xx yy and zz components separately. All three show different absorption peaks. Please suggest the correct way of representing it. I am not aware of the format obtained in the output. Please see Fig.6 in the attached reference. https://www.nature.com/articles/srep07131
It seems like an answer that was satisfactory to the OP was written in a comment by Andrey Poletayev:
"E || a should be xx, E || c should be zz. The equivalent of yy should be E || b."
This Community Wiki post can be deleted if someone wants to write a more detailed answer.