About optical absorbtion coefficient

[alok_shukla1]

Dear Experts,
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

[martin.schlipf]

I assume the difference is a rotation from the Cartesian coordinate axes (x, y, z) to one parallel to the crystal lattice vectors (a, b, c). In general, these two coordinate systems are not identical though in special cases (orthorhombic, tetragonal, cubic) they are. In higher symmetry structures there is also often the convention to show only nontrivial components. So because x, y, and z are the same in a cubic symmetry, you would only check one of them. Please analyze the symmetry of your structure carefully and see what is appropriate. You may also have to symmetrize the structure before running VASP if you did not already do so.

[alok_shukla1]

Dear Expert,
Thank you so much for this explanation and for clarifying my doubts.
I checked the symmetry of my structure; it is orthorhombic.