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Electronic properties of silicon using QE

Abdul Muhaymin

Graduate student, MSN, Bilkent University

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Outline

  • Quantum ESPRESSO input file preparation
  • Silicon structure
  • SCF
  • NSCF
  • DOS
  • Band structure
  • General electronic structure 

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Quantum ESPRESSO input file preparation

&CONTROL�  ...�/�&SYSTEM�  ...�/�&ELECTRONS�  ...�/�ATOMIC_SPECIES� X  Mass_X  PseudoPot_X� Y  Mass_Y  PseudoPot_Y�ATOMIC_POSITIONS { alat | bohr | angstrom | crystal }�  X 0.0  0.0  0.0�  Y 0.5  0.0  0.0�K_POINTS { tpiba | automatic | crystal | gamma }�if (gamma)�   nothing to read�if (automatic)�   nk1, nk2, nk3, k1, k2, k3

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Input file has two parts: namelists and namecards. Always keep the documentation open.

https://www.quantum-espresso.org/Doc/INPUT_PW.html 

Parameters to control the calculation run 

Parameters to control electronic self-consistency

List of all types of atoms in the system 

Atomic positions in specified coordinate

Which k-points to consider?

Structural information of the system of interest

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Silicon

  • FCC lattice structure (conventional unit cell), rhombohedral (primitive unit cell)
  • 8 atoms in the conventional cell
  • 2 atoms in the primitive cell
  • CIF from Materials project 
  • Visualize using XCrySDen
  • This is given as it is but we can find the structure too! (will be done tomorrow)

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SCF calculation

  • Change some parameters such as outdir, prefix, pseudo_dir
  • This is the primitive step
  • Where to find the pseudopotentials? - here
  • How do we choose other parameters? �- convergence tests! (day 2)
  • All commands do the same:
    • pw.x -i in_file > out_file
    • mpirun -np 2 pw.x < in > out
    • pw.x -in in |tee out
    • Here mpirun is used for parallel execution. 

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Density of states

  • First, get the ground state with coarse parameters (scf calculation)
    • pw.x -i scf_in_file > scf_out_file
  • Then do a non-scf calculation on the ground state with fine parameters
    • pw.x -i nscf_in_file > nscf_out_file
  • Then run the dos.x post-processing: 
    • dos.x -i dos_in_file > dos_out_file
  • Will generate a data file containing DOS info
  • Plot the `Si.dos.dat` data file 

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Band structure

  • First, get the ground state with coarse parameters (scf calculation)
  • Then do a non-scf calculation on the ground state with fine parameters
  • Then do the 'bands' calculation with high symmetry points. (bnscf input file)
  • Then run the bands.x post-processing: 
    • mpirun -np 2 bands.x < in > out
  • Will generate several data files containing band structure info
  • Plot 'Si.bands.dat.gnu' using gnuplot or python or 'Si.bands.dat' using plotband.x 

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But what can we say from the results?

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  • Can we use it in solar cell?
  • Is it a direct band gap semiconductor or an indirect band gap semiconductor? 
  • Look at the band gap! 
  • Notice the shift

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But what can we say from the results?

  • Band gap can also be seen from the DOS
  • PDOS are very helpful. Total DOS are not that much.

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Don’t forget about your first HW!

  • Go to materials-project and create an account, if you don't have any
  • Download CIF files for [[your material]]
  • HW 1: abdulmuhaymin.github.io/Workshop-2025/hw/#homework-1
  • Use QE input generator (works with POSCAR too)�https://www.materialscloud.org/work/tools/qeinputgenerator 
  • Plotting scripts are given in practice materials. Modify them accordingly or use your own plotting script and style. Python is probably the best choice (Matplotlib/plotly/bokeh) or you may use gnuplot or xmgrace.
  • All HW materials are insulators. If you still have time after this, try Aluminum or any other metallic (but non-magnetic) crystal but be careful since metals need smearing to converge.

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