Non self-consistent calculations: Band structures and Density Of States
Prev: LabQSM#Lecture 1: Basic DFT calculations and Convergences
Non self-consistent calculations: Intro
Once the charge density has been obtained in a scf run, more calculations can be performed at fixed density (non-scf). Among these:
- calculations of Kohn-Sham energies and orbitals on any given set of k-points (i.e. without specific aim of represent a reasonable sampling of the BZ);
- calculations as above, also increasing the number of computed (= diagonalized) bands (incl the calculation of empty Kohn-Sham states);
- Importantly, nscf calculations are typically done to plot KS bands or compute the DOS,
- but also as a preliminary step to other calculations (e.g. uniform k-point samplings and large amounts of empty states are using in GW and other many-body perturbation theory algorithms implemented with sum-over-states techniques)
In the Quantum ESPRESSO input file one has to set:
calculation="nscf" # kpts sampling BZ
or
calculation="bands" # kpts along lines
K-points (and number of bands if relevant) need to be specified according to the specific needs. See below for bands and DOS.
DOS
In this case a uniform k-point sampling is needed, similarly to scf calculations.
A proper sampling of the BZ is then performed, but the density is not updated.
Input example (pw.x executable) to prepare for DOS calculations:
[...]
K_POINTS {automatic}
20 20 20 0 0 0
By doing this we are de fact performing a refinement of the BZ sampling.
Setting calculation="nscf" is a good choice.
Band structure
Input example (pw.x) to obtain band data:
K_POINTS {crystal_b} 6
0.500 0.500 0.000 40 # X 0.000 0.000 0.000 40 # G 0.500 0.500 0.500 40 # L 0.750 0.500 0.250 40 # W 0.750 0.375 0.375 40 # K 0.000 0.000 0.000 1 # G
Input example (pw.x) to obtain dos data: K_POINTS {automatic} 20 20 20 0 0 0