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Does anyone know in which of the DOS output files I can find the band gap energy value of my material? I'm using Quantum ESPRESSO.

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1 Answer 1

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If you are treating your material as a non-metal (i.e. not using any smearing and using occupations='fixed'), then you can get the band gap value directly from the output of the SCF calculation. You need to use the nbnd switch in the SCF input file and then in the output file, you will have a line something like:

 highest occupied, lowest unoccupied level (ev):    5.1143 5.8455

The difference between them is the band gap. For example, in the above-mentioned case, the band gap is $(5.8455 - 5.1143) \mathrm{\;eV} = 0.7312 \mathrm{\;eV} $.

However, in case you treated your material as a metal (i.e. you used occupations='smearing') to achieve the convergence faster whereas it is actually a material with a band gap, then from inspecting the DOS output file, you can extract the band gap value.

First, open the data file produced by the dos.x calculation. The name of the data file is set in the input file as the value of the fildos switch. If you didn't define the name of the file explicitly, then by default look for the prefix.dos file. It will contain three columns for a spin-unpolarized (four for spin-polarized) calculation with the first row being:

#  E (eV)   dosup(E)     dosdw(E)   Int dos(E) EFermi =    5.670 eV

Around the Fermi level value, you will notice all the subsequent dos values are zero. Note the energy values where the first zero dos value occurs. Then scroll below further and note where the first non-zero dos value occurs again. Take the difference between these two energy value which will give you the band gap. For example, if I have a file like:

#  E (eV)   dosup(E)     dosdw(E)   Int dos(E) EFermi =    5.670 eV
...
...
   5.768  0.1976E-01  0.2202E-01  0.5840E+03
   5.778  0.3176E-02  0.3274E-02  0.5840E+03
   5.788  0.7654E-03  0.8099E-03  0.5840E+03
   5.798  0.1230E-04  0.1696E-04  0.5840E+03
   5.808  0.0000E+00  0.0000E+00  0.5840E+03
   5.818  0.0000E+00  0.0000E+00  0.5840E+03
   5.828  0.0000E+00  0.0000E+00  0.5840E+03
   5.838  0.0000E+00  0.0000E+00  0.5840E+03
...
...
   7.408  0.0000E+00  0.0000E+00  0.5840E+03
   7.418  0.0000E+00  0.0000E+00  0.5840E+03
   7.428  0.0000E+00  0.0000E+00  0.5840E+03
   7.438  0.0000E+00  0.0000E+00  0.5840E+03
   7.448  0.0000E+00  0.0000E+00  0.5840E+03
   7.458  0.1580E-04  0.1575E-04  0.5840E+03
   7.468  0.9316E-04  0.9306E-04  0.5840E+03
   7.478  0.2350E-03  0.2348E-03  0.5840E+03
...
...

The band gap is in this case: $(7.458 - 5.808) \mathrm{\;eV} = 1.95 \mathrm{\;eV} $.

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