Phonon dispersions with Density-Functional Perturbation Theory
make ph pp tools("make all" is also ok).
tar -zxvf examples_disp.tgz cd examples_dispIn the following, $espresso_dir stands for the root directory of the Quantum ESPRESSO distribution.
Interatomic Force Constants in Real SpaceWhenever the entire phonon dispersion is needed, it is convenient to calculate Interatomic Force Constants (IFC) in real space. This calculation involves three steps:
$espresso_dir/bin/pw.x < si.scf.in > si.scf.out
Sample input file: si.ph.in. One has to specify the grid of q-vectors as in a Monkhorst-Pack grid, with variables "nq1", "nq2", "nq3". The grid must contain Gamma and it must be a subdivision of the reciprocal lattice (i.e. it must correspond to a supercell in R-space). It is automatically generated. One has also to specify "ldisp=.true." to instruct the code to loop over all q-vectors.
$espresso_dir/bin/ph.x < si.ph.in > si.ph.outNote that for each q-vector the dynamical matrices are saved with a different name ("fildyn" + 1-8), while "fildyn" + 0 contains the information on the q-vector grid (type of grid and number of points)
This task is performed by auxiliary program "q2r.x". All dynamical matrices are read and Fourier-transformed. The input file for q2r.x contains very little data: file name "fildyn" of dynamical matrices (the same given in input to the phonon code), whether to apply ASR (better to do it: asr='simple' or 'crystal' are ok for simple crystals), the name ("flfrc") of the output file containing the force constants.
$espresso_dir/bin/q2r.x &input fildyn='si.dyn', zasr='simple', flfrc='si444.fc' /
Phonon Dispersions from Interatomic Force ConstantsOnce the IFC's in real space are available, one can calculate phonons at any value of q using code "matdyn.x" (do not confuse with "dynmat.x"). It can calculate phonon Density Of States (DOS) as well. Before starting more serious calculations, it is wise to check whether the quality of the Fourier interpolation performed in the previous step is sufficiently good, i.e. if you need a denser q-vector grid (i.e. a longer range for the IFC's) or not. This can be easily checked by comparing the results of matdyn.x and of ph.x for a q-vector that was not in the grid used in the Fourier interpolation. Once you are convinced that your IFC set is ok, let us proceed to two different types of calculations;
Sample file: si.matdyn.in. You must specify the name of the file ("flfrc") containing the IFC's, and a list of q-points for which the frequencies are to be calculated. You may specify various forms of ASR ('simple' or 'crystal' are ok in this case), where to write the frequencies ("flfrq").
$espresso_dir/bin/matdyn.x < si.matdyn.inThe file selected in flfrc, "si.freq", contains a list of frequencies in a format that can be further processed by another auxiliary code, "plotband.x", the same used for band structure plotting:
$espresso_dir/bin/plotband.x si.freqAnswer something sensible for Emin and Emax (0 530 for instance; units are cm-1) and for the file in "xmgr" format, for instance, "siph.xmgr"; you may also produce a file directly in printable (postscript) format. The file "siph.xmgr" can be easily plotted using xmgrace or gnuplot plotting programs. Does it look like you expected it?
If you choose option "dos=.true." you must specify the grid of q-vectors used to calculate the DOS (variables "nk1", "nk2", "nk3", using the Monkhorst-Pack logic). You should also specify where you want the DOS written ("fldos") Sample file: si.phdos.in.
$espresso_dir/bin/matdyn.x < si.phdos.in > si.phdos.outThe file selected in fldos, "si.phdos", contains the DOS in a format that can be plotted using xmgr, xmgrace, or gnuplot. Can you make the connection between the phonon dispersions with the phonon DOS?
Polar MaterialsThe calculation of IFC's in polar materials is basically the same as for nonpolar materials. You just have to remember to specify the appropriate option ("epsil=.true.") in the input of trhe phonon code. Can you produce a phonon dispersion plot for AlAs?
Phonons with Ultrasoft pseudopotentialsPhonon calculations with Ultrasoft pseudopotentials are basically the same as for norm-conserving ones. You just have to be aware of the following tricks:
Phonons in magnetic systemsThe example for Ni shows how to calculate phonon dispersions in a magnetic system (incidentally one that is also metallic and with ultrasoft pseudopotentials). The procedure is the same: self-consistency (pw.x) first (sample data: ni.scf.in), followed by the phonon calculation (ph.x) (sample data: ni.ph.in) and by inverse Fourier transform (q2r.x) to get the IFC's in real space (sample data: ni.q2r.in). Once you have the IFC's, you can plot the phonon dispersions using "matdyn.x" (sample data: ni.matdyn.in) and "plotband.x".