In this tutorial, We will see how to calculate the DOS and the Band Structure in the same directory and how to combine them to get one plot using the code SUMO.

First, we need to install the sumo code as following: 

$ pip install sumo

 

We need 2 files:

Si.cell

%block lattice_cart
 0.000000  2.715000  2.715000
 2.715000  0.000000  2.715000
 2.715000  2.715000  0.000000
%endblock lattice_cart

%block positions_abs
Si  0.000000  0.000000  0.000000
Si  1.357500  1.357500  1.357500
%endblock positions_abs

kpoints_mp_grid 7 7 7

symmetry_generate

 

Si.param

task                singlepoint
xc_functional       pbe
max_scf_cycles      100
basis_precision     fine
elec_energy_tol     1e-7
elec_eigenvalue_tol 1e-6
nextra_bands        8

 

Now run a singlepoint calculation with e.g.

mpirun -np 4 castep.mpi si     # for parallel calculation with 4 processors

This calculation writes eigenvalue data to si.bands. We can obtain a rough DOS by plotting this file with Sumo:

sumo-dosplot --code castep -f si.bands --format PNG

 


This writes a file dos.png containing a rough DOS plot. However, it doesn’t look very good; we see discrete spikes at the eigenvalues. To obtain a smooth spectrum we can add broadening; try 0.1 eV using

sumo-dosplot --code castep -f si.bands -g 0.1 --format PNG 


 (the “-g” is for “Gaussian”). The result looks a lot better, but is still suspiciously spiky. We need more data points, but our 7x7x7 k-point grid should have been sufficient for a good SCF. To get more DOS points, we reboot CASTEP with a “spectral” task to compute eigenvalues non-self-consistently on a fine k-point mesh.

Create a modified cell file si-dos.cell, using SPECTRAL_KPOINTS_MP_GRID to set a fine mesh.

%block lattice_cart
 0.000000  2.715000  2.715000
 2.715000  0.000000  2.715000
 2.715000  2.715000  0.000000
%endblock lattice_cart

%block positions_abs
Si  0.000000  0.000000  0.000000
Si  1.357500  1.357500  1.357500
%endblock positions_abs

kpoints_mp_grid 7 7 7
spectral_kpoints_mp_grid 21 21 21

symmetry_generate

In the si-dos.param file we set up a spectral DOS task and REUSE the previous calculation results to avoid unnecessary SCF work. We might also crank up the number of bands to allow more empty states to be created. Strictly, one should not attach much meaning to empty states from Kohn-Sham DFT calculations; in practice, they are very useful.

task                      spectral
spectral_task             dos
reuse                     si.check
xc_functional             pbe
max_scf_cycles            100
basis_precision           fine
elec_energy_tol           1e-7
elec_eigenvalue_tol       1e-6
spectral_perc_extra_bands 50

and run with

mpirun -np 4 castep.mpi si-dos  # # for parallel calculation with 4 processors

If we check the si-dos.castep output we see that the SCF converged very quickly. Most of the run time was then spent evaluating spectral k-points. Plotting our new .bands file with Sumo, the DOS should be a lot more complete.

sumo-dosplot --code castep -f si-dos.bands -g 0.1 --format PNG

 


We can get more detail about the band edges using a band structure calculation. In CASTEP the procedure is similar to a spectral DOS calculation, but we need to specify the paths of k-points to explore. It’s a bit tedious to create these by hand, so we’ll let Sumo do it for us.

sumo-kgen --code castep -p si.cell

creates a band.cell file with the k-point path included. If you look in the file you can see that the other settings of the .cell file have been retained. The spectral k-point list includes some comments to mark high-symmetry points; this information will be used later for plotting. For now we make a band.param file which is identical to the DOS parameters except for the SPECTRAL_TASK:

task                      spectral
spectral_task             bandstructure
reuse                     si.check
xc_functional             pbe
max_scf_cycles            100
basis_precision           fine
elec_energy_tol           1e-7
elec_eigenvalue_tol       1e-6
spectral_perc_extra_bands 50

mpirun -np 4 castep.mpi band

The key data is written to band.bands and can be plotted with

sumo-bandplot --code castep -f band.bands --format PNG

 


which generates a publication-ready band.pdf including special-point labels read from the .cell file.

We can simultaneously read in the DOS data for a combined plot:

sumo-bandplot --code castep -f band.bands --dos si-dos.bands -g 0.1 --format PNG


 

 

 Reference:  https://github.com/ajjackson/castep-sumo-tutorial/blob/master/castep-sumo.org