Phonons & Phonopy: Pro Tips

Transcription

1 Phonons & Phonopy: Pro Tips [Version 2.0] J. M. Skelton Bath CompChem 14 th April 2015

2 Phonons and La*ce Dynamics Crystallography is generally concerned with the sta3c proper3es of crystals, describing features such as the average posi3ons of atoms and the symmetry of a crystal. Solid state physics takes a similar line as far as elementary electronic proper3es are concerned. We know, however, that atoms actually move around inside the crystal structure, since it is these mo3ons that give the concept of temperature [ ]. The sta3c labce model, which is only concerned with the average posi3ons of atoms and neglects their mo3ons, can explain a large number of material features [ ]. There are, however, a number of proper3es that cannot be explained by a sta3c model MarEn Dove, IntroducEon to LaNce Dynamics Bath CompChem, April 2015 Slide 2

3 What Can Phonopy Do? Anharmonicity phonopy Proper:es phonopy-gruneisen phonopy-qha phono3py Bath CompChem, April 2015 Slide 3

4 Overview PracEcal Theory phonopy Workflow/setup CalculaEng forces: opeons and gotchas Post processing Input/output files; using phonopy with other force- constant calculators phonopy-qha Workflow and post- processing Example applicaeons: which DFT funceonal? phono3py Workflow, setup and post processing Examples Some current developments Summary Bath CompChem, April 2015 Slide 4

5 Prac:cal Theory Force- constant matrix: From finite differences: Dynamical matrix: A_er diagonalisaeon: Bath CompChem, April 2015 Slide 5

6 Prac:cal Theory Bath CompChem, April 2015 Slide 6

7 phonopy: Workflow Input Structure Create Displacements phonopy -d [--dim= ] Calculate Forces Extract Forces phonopy -f vasprun-{001..xxx}.xml phonopy --fc vasprun.xml Post- Process phonopy [-t] [-p] [-s] Settings.conf Bath CompChem, April 2015 Slide 7

8 phonopy: Setup phonopy -d [--dim= ] [--tolerance=1e-5] [-c POSCAR] Set up the calculaeons on an XxYxZ supercell PosiEon tolerance for symmetry deteceon Path to POSCAR- format structure ( cell ), if not POSCAR disp.yaml POSCAR-001 POSCAR-002 POSCAR SPOSCAR InformaEon about the structure, supercell and displaced atoms POSCAR files containing single atomic displacements Unperturbed supercell for DFPT phonon calculaeon Bath CompChem, April 2015 Slide 8

9 phonopy: Calcula:ng Forces Sample finite- displacement INCAR: ADDGRID =.TRUE. EDIFF = 1E-8 ENCUT = ev LREAL =.FALSE. PREC = High Accurate Sample VASP force- constants INCAR: ADDGRID =.TRUE. EDIFF = 1E-8 ENCUT = ev IBRION = LREAL =.FALSE. NSW = 1 PREC = High Accurate Accurate forces are esseneal - > crank the standard senngs right up LREAL =.FALSE. is esseneal, unless you manually adjust ROPT ADDGRID =.TRUE. doesn t seem to be esseneal, but doesn t cost much either For finite- difference/dfpt phonon calculaeons in VASP, set NSW = 1 Bath CompChem, April 2015 Slide 9

10 phonopy: Calcula:ng Forces (PbTe) ADDGRID =.FALSE. LREAL =.FALSE. ADDGRID =.FALSE. LREAL = Auto ADDGRID =.TRUE. LREAL =.FALSE. ADDGRID =.TRUE. LREAL = Auto Bath CompChem, April 2015 Slide 10

11 phonopy: Calcula:ng Forces (ZnS) Bath CompChem, April 2015 Slide 11

12 phonopy: Calcula:ng Forces (ZnS) Symmetrising the force constants during the post processing with phonopy corrects the PBE dispersion SymmetrisaEon enforces the acousec- sum rule in the TPSS dispersion, but does not remove the artefacts away from Γ Symmetrising within phonopy gives the same result as calcula3ng the force constants with the VASP internal DFPT/finite- displacement rou3nes - as with most codes, VASP enforces symmetry by default Bath CompChem, April 2015 Slide 12

13 phonopy: Calcula:ng Forces (ZnS) Bath CompChem, April 2015 Slide 13

14 phonopy: Calcula:ng Forces Bath CompChem, April 2015 Slide 14

15 phonopy: Calcula:ng Forces Bath CompChem, April 2015 Slide 15

16 phonopy: A Few Top Tips If using VASP FD/DFPT, set NWRITE = 3 in the INCAR file, and you can run this bash script on the OUTCAR to obtain a simulated IR spectrum for free : hrp://homepage.univie.ac.at/david.karhanek/downloads.html#entry02 If using DFPT with an LDA/GGA funceonal, set LEPSILON =.TRUE. in the INCAR file to obtain the staec dielectric constant, in parecular the ionic- relaxaeon part, for a small added cost When using FD/DFPT, VASP tries to change the k- point set internally, which requires NPAR = #Cores to be set in the INCAR file; senng ISYM = -1 avoids this, and although the number of displacements which need to be evaluated may increase, the performance gained by using band parallelism can quite easily offset this for low- symmetry systems (!) Bath CompChem, April 2015 Slide 16

17 phonopy: Post Processing phonopy -p -s Settings.conf SeNngs file Plot Save PbTe Sample phonon DOS se*ngs file: DIM = MP = GAMMA_CENTER =.TRUE. Bath CompChem, April 2015 Slide 17

18 phonopy: Post Processing phonopy -p -s Settings.conf SeNngs file Plot Save PbTe Sample phonon DOS se*ngs file: DIM = MP = GAMMA_CENTER =.TRUE. EIGENVECTORS =.TRUE. PDOS = 1, 2 Bath CompChem, April 2015 Slide 18

19 phonopy: Post Processing phonopy -p -s -t Settings.conf SeNngs file Plot Save [Calculate] thermal properees PbTe Sample phonon DOS se*ngs file: DIM = MP = GAMMA_CENTER =.TRUE. Bath CompChem, April 2015 Slide 19

20 phonopy: Post Processing phonopy -p -s Settings.conf SeNngs file Plot Save PbTe Sample phonon band structure se*ngs file: DIM = BAND = BAND_POINTS = 101 BAND_LABELS = \Gamma W X \Gamma L [EIGENVECTORS =.TRUE.] Bath CompChem, April 2015 Slide 20

21 phonopy: Post Processing phonopy -p -s Settings.conf SeNngs file Plot Save PbTe Sample phonon band structure se*ngs file: DIM = BAND = BAND_POINTS = 101 BAND_LABELS = \Gamma W X \Gamma L BAND_CONNECTION =.TRUE. Bath CompChem, April 2015 Slide 21

22 phonopy: Non- Analy:cal Correc:ons To apply a non- analyecal correceon (LO/TO splinng) to the phonon frequencies, phonopy needs the Born effeceve charges and electronic- polarisaeon contribueon to the macroscopic dielectric constant In VASP, for LDA/GGA funceonals these can be computed using DFPT; for others, they need to be computed from the response to an electric field INCAR for Born charges using DFPT: EDIFF = 1E-8 ENCUT = ev LEPSILON =.TRUE. LREAL =.FALSE. NSW = 0 PREC = High Accurate INCAR for Born charges using LCALCEPS: EDIFF = 1E-8 ENCUT = ev LCALCEPS =.TRUE. LREAL =.FALSE. NSW = 0 PREC = High Accurate [EFIELD_PEAD = Ex Ey Ez] Bath CompChem, April 2015 Slide 22

23 phonopy: Non- Analy:cal Correc:ons outcar-born > BORN Sample BORN file: <Conversion Factor> ε xx ε xy ε xz ε yx ε yy ε yz ε zx ε zy ε zz Z xx Z xy Z xz Z yx Z yy Z yz Z zx Z zy Z zz Z xx Z xy Z xz Z yx Z yy Z yz Z zx Z zy Z zz Dielectric tensor Born charge tensors for unique atoms CorrecEons are enabled by senng NAC =.TRUE. in the configuraeon file, or passing --nac as a command- line argument When this opeon is used, phonopy expects to find a BORN file in the working directory Bath CompChem, April 2015 Slide 23

24 phonopy: Non- Analy:cal Correc:ons NAC =.FALSE. NAC =.TRUE. PbTe Bath CompChem, April 2015 Slide 24

25 phonopy: Force- Constant Symmetrisa:on ZnS FC_SYMMETRY = 0 FC_SYMMETRY = 1 Force- constant symmetrisaeon is enabled by senng FC_SYMMETRY = > 0 in the configuraeon file Bath CompChem, April 2015 Slide 25

26 phonopy: Output Files Sample mesh.yaml file: Sample band.yaml file: mesh: [ m x, m y, m z ] nqpoint: natom: 8 phonon: - q-position: [ q x, q y, q z ] weight: w 1 band: - # 1 frequency: ω 1... nqpoint: 808 npath: 8 natom: 8 phonon: - q-position: [ q x, q y, q z ] distance: d 1 band: - # 1 frequency: ω 1... If EIGENVECTORS =.TRUE. is set in the configuraeon file, the mode eigenvectors will also appear in these files With BAND_CONNECTION =.TRUE., the frequencies for each band in band.yaml are ordered so that they connect across the band structure Bath CompChem, April 2015 Slide 26

27 phonopy: Output Files Sample total_dos.dat file: # Sigma = The pareal_dos.dat file generated with EIGENVECTORS =.TRUE. contains one column for each atom in the primieve cell Sample thermal_proper:es.yaml file: unit: temperature: K... natom: 8 zero_point_energy: high_t_entropy: thermal_properties: - temperature: free_energy: entropy: heat_capacity: energy: Bath CompChem, April 2015 Slide 27

28 phonopy: Force- Constant Calculators Sample FORCE_SETS file: d 1x d 1y d 1z F 1x F 1y F 1z F 2x F 2y F 2z... 2 d 2x d 2y d 2z F 1x F 1y F 1z F 2x F 2y F 2z... Sample FORCE_CONSTANTS file: Φ xx Φ xy Φ xz Φ yx Φ yy Φ yz Φ zx Φ zy Φ zz 1 2 Φ xx Φ xy Φ xz Φ yx Φ yy Φ yz Φ zx Φ zy Φ zz... Bath CompChem, April 2015 Slide 28

29 phonopy: Force- Constant Calculators IRMOF- 10 *Tinker calculaeons by J. K. Bristow and D. Tiana Bath CompChem, April 2015 Slide 29

30 phonopy-qha: Workflow EoS Curve E/V curve - > e- v.dat Harmonic Phonopy phonopy -d --dim=... phonopy -f vasprun-{001..xxx}.xml Thermal ProperEes phonopy -t -p -s Settings.conf Post- Process Needs to be two points smaller than the maximum temperature in the YAML files phonopy-qha e-v.dat thermal_properties-{001..xxx}.yaml --tmax=980 [--tstep=10] [--pressure=<p/gpa>] [-p] [-s] Bath CompChem, April 2015 Slide 30

31 phonopy-qha: Output *J. M. Skelton et al., Phys. Rev. B 89, (2014) Bath CompChem, April 2015 Slide 31

32 phonopy-qha: Output bulk_modulus- temperature.dat <- B is temperature dependent(!) Cp- temperature.dat Cp- temperature_polyfit.dat Cv- volume.dat <- C V at each volume, at each temperature dsdv- temperature.dat entropy- volume.dat <- S V at each volume, at each temperature gibbs- temperature.dat gruneisen- temperature.dat helmholtz- volume.dat thermal_expansion.dat <- Average Gruneisen parameter (?) <- A at each volume, at each temperature volume_expansion.dat volume- temperature.dat Bath CompChem, April 2015 Slide 32

33 phonopy-qha: Examples *J. M. Skelton et al., in preparaeon Bath CompChem, April 2015 Slide 33

34 phonopy-qha: Examples *J. M. Skelton et al., in preparaeon Bath CompChem, April 2015 Slide 34

35 phonopy-qha: Examples *J. M. Skelton et al., in preparaeon Bath CompChem, April 2015 Slide 35

36 phonopy-qha: Examples ~4 K ~105 K ~150 K K K K *J. M. Skelton et al., Phys. Rev. B 89, (2014) Bath CompChem, April 2015 Slide 36

37 phonopy-qha: Examples SnS: 10 K K PbTe: 0 K K *PbTe: J. M. Skelton et al., Phys. Rev. B 89, (2014) Bath CompChem, April 2015 Slide 37

38 phono3py: Workflow Input Structure Create Displacements phono3py -d --dim= Calculate Forces Extract Forces phono3py --cf3 vasprun-{001..xxx}.xml Post- Process phono3py --dim= phono3py --tmax= tstep=10 --fc2 --fc3 --dim= v --mesh= br --sigma= 0.1 Bath CompChem, April 2015 Slide 38

39 phono3py: Setup phono3py -d --dim=... [--cutoff_pair=4] [--dim_fc2= ] phono3py --cf3 vasprun-{001..xxx}.xml Extract forces from VASP output phono3py --dim=... [--dim_fc2...] The --cutoff_pair tag uses the same numbering for the displaced POSCAR files as the full calculaeon; this means the cutoff can be increased, and the extra displacements added, systemaecally, to converge w.r.t. the interaceon range Bath CompChem, April 2015 Slide 39

40 phono3py: Post Processing Read in pre- calculated force constants phono3py --tmax= tstep=10 --fc2 --fc3 --dim=... -v --mesh= br --sigma= 0.1 [--nac] [--dim2=... /--dim_fc2=... ] # of interaceons per q- point becomes larger with mesh size; cannot easily max out as for DOS calculaeons, but needs to be converged The post- processing (mainly the phonon- lifeeme calculaeons) takes a very long Eme for large supercells/large or low- symmetry structures It is possible to run the calculaeon on (ranges of) q- points separately, and then combine them a_erwards Various post- processing tags can be applied, e.g. to incorporate isotope effects Bath CompChem, April 2015 Slide 40

41 phono3py: Examples *J. M. Skelton et al., APL Materials 3, (2015) Bath CompChem, April 2015 Slide 41

42 phono3py: Examples Simulated IR/Raman spectra with 10 K linewidths 300 K linewidths and an instrument broadening of 3.5 cm - 1, compared to expt. *J. M. Skelton et al., APL Materials 3, (2015) Bath CompChem, April 2015 Slide 42

43 Some Current Developments *E. L. da Silva et al., Phys. Rev. B 91, (2015) Bath CompChem, April 2015 Slide 43

44 A Few Closing Remarks Bath CompChem, April 2015 Slide 44