View source for Solution LAB1 bands DOS diamond


* Back to the previous page: [[Non self-consistent calculations: Band structures and Density Of States#Examples and Exercises]]

=== Preliminaries ===

Given the calculations run previously to determine the converged set of k-points, ecutwfc, and lattice parameter, here we set the following:

  nk = 8
  ecutwfc = 60.0     # could be lowered further
  alat = 6.6694212

As a preliminary step:
* make a new directory, e.g. called Diamond_BANDS
* In a situation like this it is always a good idea to start over from scratch with a fresh scf calculation.
* prepare an input file for a single scf run according to the parameters above.
  &CONTROL
    calculation="scf"
    [...]
  /
  &SYSTEM
    alat=...
    ecutwfc=...
  /
  [...]
  K_POINTS {automatic}
  8 8 8  0 0 0

* run the calculation as:
  mpirun -np 2 pw.x < scf.in > scf.out
* The use of <code>mpirun -np 2</code> in the example above makes the calculation running on two MPI processes.

=== DOS of Diamond ===
We start by addressing the '''calculation of the DOS'''. 

We need now to write the input file for a non-self consistent calculation using a regular k-point grid.
The input file looks like:
  >$ cat nscf_dos.in
  &CONTROL
    calculation="nscf"
    verbosity="high"     # not strictly needed but useful sometimes
    [...]
  /
  &SYSTEM
    alat=...
    ecutwfc=...
    nbnd=20              # let's include some empty states
  /
  [...]
  K_POINTS {automatic}
  20 20 20  0 0 0        # here we use a much denser kpt grid

Then we run it
  mpirun -np 2 pw.x < nscf_dos.in > nscf_dos.out

Given the number of k-points requested, the calculation may take some time.

At this point we can prepare an input file for the <code>dos.x</code> program, 
  $> cat  dos.in
  &DOS
     prefix = 'diamond'
     outdir = './SCRATCH/'    
     degauss=0.025  !  [Ry]  broadening parameter
     fildos = 'dos_diamond_0.025Ry.dat'
  / 

The variable <code>degauss</code> controls the gaussian broadening used to approximate 
Dirac delta's in the calculation. 

Try to change this value to get a feeling of the effect is has on the computed DOS.

An example is show in the figure below.

[[File:Dos_diamond.jpg | 450px| DOS of Diamond for different values of gaussian broadening.]]


=== Band structure of Diamond ===

In order to '''compute a band structure''' we need to run a dedicated nscf calculation.

This will overwrite previous nscf calculations, but does not modify the previously compute density.

We need now to write the input file for a non-self consistent calculation using a regular k-point grid.
For the sake of convenience, a plot of the zinc-blende Brillouin zone (and related conventions) is provided below.

[[File:Common-Brillouin-zone-of-diamond-and-zinc-blende-semiconductors Q640.jpg|400px]]

The [https://www.materialscloud.org/work/tools/seekpath MaterialsCloud SeekPath] tool can be also used to inspect the BZ.


'''The <code>pw.x</code> input file''' should look like the following:
  >$ cat nscf_bands.in
  &CONTROL
    calculation="bands"
    verbosity="high"     # not strictly needed but useful sometimes
    [...]
  /
  &SYSTEM
    alat=...
    ecutwfc=...
    nbnd=20              # let's include some empty states
  /
  [...]
  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

We run it as before
  mpirun -np 2 pw.x < nscf_bands.in > nscf_bands.out

Next, bands can be reformatted for plotting using the <code>bands.x</code> provided by Quantum ESPRESSO.

Alternatively, the home-made tool
  ./LabQSM/tools/plot_bands.py  nscf_bands.out
can be used.
  
The resulting plot is reported below.

[[File:Bands diamond.jpg|600px|Diamond bands]]