My Community

Hi,

I am a new VASP user. I am trying to calculate the formation energy of a defect and I always come across the term "Valence Band Maximum" (VBM). How can I calculate the VBM using VASP??

Cheers,
Lexa

any help??

Hi Lexa,
In order to calculate VBM and CBM, you need to perform band structure calculation over a set of high symmetry k-path points, using line mode. This will give you a plot of VB (as well as other bands) energy (eigenvalue) in k-space, from which you can find VBM.

However, I am not certain what you are referring about the connection between defect formation energy and VBM. Although, certainly the energies of VB and CB contribute to formation energy, all occupied bands should be taken care of. Therefore, the formation energy should be rather estimated from the difference in chemical potential of the defect system and that of pure system. Chemical potential is the total energy per formula unit minus the total energies of its constituent atoms. Depending on your particular system of interest, you may treat the constituent atoms as isolated atomic system (i.e. one atom in large box) or dimer/molecule (diatom/molecule in a large box) or periodic system.

Please note: total energy is not sum of the occupied band energies.

Hope this info helps.
Cheers,
Sankh

Many thanks Sankh... In that case isnt it easy just to grep 'E-fermi' OUTCAR which will give you the fermi energy? or am I wrong??!

Cheers