DFT Calculations with Quantum Espresso: User Guide

1. DFT Calculations with Quantum Espresso: User Guide Janam Jhaveri Network for Computational Nanotechnology (NCN) Electrical and Computer Engineering jjhaver@purdue.edu 02/07/2011

2. GettingStarted

3. Quick Run: DOS and Band Structure Silicon

4. Quick Initial Run Hit ‘Simulate’ and wait for results

5. Quick Initial Run • Under Results you should see: • Density of States plot • Band Structure plot • SCF Output Log • Energy Evolution Plot • Energy, forces and stress data • Input files for quantum espresso executables

6. Input options explained

7. Input Geometry

8. Input Geometry Atomistic Structure • Optional: Select whether to upload input geometry or use a pre-made structure

9. Input Geometry Atomic Coordinates • Select whether to use Cartesian or Fractional coordinates Cartesian coordinates: coordinates given in Angstrom using a Cartesian coordinate system (x,y,z) Fractional coordinates: coordinates given in terms of unit cell’s lattice vectors, value must be between 0.0 and 1.0

10. Input Geometry Structure Type • Select structure type • See p.11-12 for more info

11. Input Geometry Structure types Simple Cubic (cubic P) Face-Centered Cubic (cubic F) http://en.wikipedia.org/wiki/Crystal_structure

12. Input Geometry Structure types Body-Centered Cubic (cubic I) Hexagonal (Hexagonal and Trigonal P) http://en.wikipedia.org/wiki/Crystal_structure

13. Input Geometry Atomic Coordinates Enter structure (format shown below) Format: Number of atoms Information about structure Atom symbol coordinate 1 coordinate 2 coordinate 3 Atom symbol coordinate 1 coordinate 2 coordinate 3 …

14. Input Geometry Cell Vectors If structure type chosen is ‘Determine unitcell (free)’, enter cell vectors

15. Input Geometry Lattice Parameter a If structure type chosen is NOT ‘Determine unitcell (free)’, enter lattice parameter a See p. 11-13 for definition of lattice parameter a

16. Input Geometry Lattice Parameter c If structure type chosen is ‘Hexagonal and Trigonal P’, also enter the ratio between lattice parameters c and a See p. 12 for definition of lattice parameter c

17. Energy Expression

18. Energy Expression Functional Select either LDA or GGA (PBE) for Exchange-Correlation functional See p. 19 for more info

19. Energy Expression Exchange-correlation energies LDA (Local Density Approximation): approximations of exchange-correlation energies that depend only on value of electronic density at each point in space GGA (Generalized Gradient Approximation): still local, but also includes gradient of density at that point in space Quantum Espresso uses pseudopotentials to implement exchange-correlation energies. Pseudopotentials are approximations of the wavefunctions of valence electrons in the nuclear core region. This leads to a modified potential term instead of Coulombic potential term in the Schrodinger Equation. Pseudopotentials for atoms are available as specified by http://www.pwscf.org/pseudo.php

20. Energy Expression Relax Optional: Select either force or cell relaxation Force Relax: minimize     forces while keeping the unit cell boundaries fixed Cell Relax: allow the unit cell boundaries to relax

21. Energy Expression K-grid Select number of k-points to be used in the x, y and z- direction

22. Energy Expression Bands Select number of bands

23. Energy Expression Energy cutoffs Select energy cutoffs and self-consistent field (SCF) convergence criterion

24. Energy Expression Occupation Options Optional: Enable occupation options and select occupation See p. 27 for more info on occupation options

25. Energy Expression Smearing If Occupation selected is ‘smearing’, choose a smearing type See p. 28 for more info on smearing options

26. Energy Expression Gaussian Spreading If Occupation selected is ‘smearing’, enter a value for the Gaussian spreading/broadening

27. Energy Expression Occupation Options source: http://www.quantum-espresso.org/input-syntax/INPUT_PW.html#id3900903 “smearing: Gaussian smearing for metals tetrahedra: for calculation of DOS in metals (see PRB49, 16223 (1994)) Not suitable (because not variational) for force/optimization/dynamics calculations fixed: for insulators with a gap“

28. Energy Expression Smearing Options source: http://www.quantum-espresso.org/input-syntax/INPUT_PW.html#id3900938 “Gaussian: ordinary Gaussian spreading Methfessel-Paxton: Methfessel-Paxton first-order spreading (see PRB 40, 3616 (1989)) Marzari-Vanderbilt: Marzari-Vanderbilt cold smearing (see PRL 82, 3296 (1999)) Fermi-Dirac: smearing with Fermi-Dirac function”

29. Energy Expression Mixing Options Optional: Enable mixing options and select mixing mode See p. 31 for more info on mixing modes

30. Energy Expression Mixing factor Enter a value for the mixing factor

31. Energy Expression Mixing modes source: http://www.quantum-espresso.org/input-syntax/INPUT_PW.html#id3901509 “plain: charge density Broyden TF : as above, with simple Thomas-Fermi screening (for highly homogeneous systems) local-TF: as above, with local-density-dependent TF screening (for highly inhomogeneous systems)”

32. Phonons

33. Phonons Phonon Calculations Optional: select whether to perform phonon calculations

34. Phonons Calculation type Select whether to perform dispersion relationship or a single q-point calculation

35. Phonons Atomic Mass Enter list of atom types and their mass in amu (format shown below) Format: Atom symbol mass Atom symbol mass …

36. Phonons Convergence criterion Enter energy value as phonon convergence criterion. Note energy is given in Ry.

37. Phonons Dielectric constant Optional: choose to calculate macroscopic dielectric constant Warning: will not work with metallic systems

38. Phonons Q-grid spacing If you’re doing a dispersion relationship calculation, select number of q-points to be used in the x, y and z- direction

39. Phonons Dispersion curve path If you’re doing a dispersion relationship calculation, select high symmetry points (Q-points) for the path. Q-points should be given in fractional (reduced) coordinates.

40. Phonons Number of points Enter number of points on dispersion curve path

41. Phonons Single q-point If you’re doing a single Q-point calculation, choose Q-point on which to do phonon calculations

42. Band structure and Density of States

43. Band structure Band structure Calculations Optional: select whether to perform band structure calculations

44. Band structure Band structure path Select high symmetry points (K-points) along which the path should be plotted. K-points should be given in fractional (reduced) coordinates.

45. Band structure Number of points on path Enter number of points on band structure path

46. Density of States Minimum Energy Enter minimum of energy grid to plot. Note energy is given in eV.

47. Density of States Maximum Energy Enter maximum of energy grid to plot

48. Density of States Energy grid step Enter energy grid step to plot

49. References • http://en.wikipedia.org/wiki/Crystal_structure • http://www.pwscf.org/pseudo.php • http://www.quantum-espresso.org/input-syntax/INPUT_PW.html#id3900903 • http://www.quantum-espresso.org/input-syntax/INPUT_PW.html#id3900938 • http://www.quantum-espresso.org/input-syntax/INPUT_PW.html#id3901509