My structure is Orthorhombic with space group Pbcm or Pmc21. I need to find out the atoms positions for Pbcm or Pmc21. I have seen for same group different wyckoff positions in Bilbao Crystallographic Server. I have to find the band structure and density of states of my material. How should I do this?
The reason you find different sets of atomic positions for Pbcm structures is that the space group does not uniquely determine the positions of the atoms. The space group determines only the SYMMETRY of the structure. In each space group, there is a whole set of different crystal structures with the same symmetry elements but different atomic positions. Look at a space group diagram (http://img.chem.ucl.ac.uk/sgp/large/sgp.htm) and you'll see a list of equivalent positions; this is as close as it's possible to get to the answer you want!
You'll need to tell us what your material is, or search for it in a database like the Bilbao Crystallographic Server or ICSD. Once you have the atom positions for the correct material, you can start to think about calculating band structure and DoS using the DFT code of your choice.
If you have your structure in CIF format from somewhere like ICSD or COD (www.crystallography.net), then I have written cif2cell, a program that will set up the structure for a lot of electronic structure programs. You can get it from the link given below. The Bilbao Crystallographic server will also be able to generate a CIF file for you if you know your Wyckoff positions and crystallographic parameters (the 'setstru' utility).
Then what program to use to calculate the DOS is a different question. If you know nothing about electronic structure calculations, I would recommend Elk or Exciting (FP-LAPW programs) which should do anything out of the box. If faster calculations are required, the most obvious open software choice at the moment is Abinit, I think, although there you have the drawback of needing to understand (or at least test) pseduopotentials.
http://sourceforge.net/projects/cif2cell/
For what it's worth, I use SIESTA for such calculations at the moment. However, being an LCAO code, this requires understanding not just pseudopotentials but basis sets as well.
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