4.11 what='scf_disp'

With this option the code makes a self-consistent calculation followed by a phonon dispersion calculation at a fixed geometry. The geometry is given in the input of pw.x. The dynamical matrices are used to calculate the interatomic force constants. Using these interatomic force constants the code calculates the dynamical matrices and hence the phonon frequencies along a path in the Brillouin zone and on a much thicker mesh of q points. The path can be generated automatically or given in input as in a band structure calculation (see above what='scf_bands'). The uniform mesh of q points is specified in thermo_control. The code uses the phonon frequencies calculated on the thick mesh of q points to get the phonon density of states using a smearing approach. The density of states is used to calculate the harmonic thermodynamic properties: vibrational energy, vibrational free energy, vibrational entropy, and constant strain heat capacity. The same thermodynamic quantities are calculated also by direct integration over the Brillouin zone and compared in the plots. When with_eigen=.TRUE. the atomic B-factors are calculated as a function of temperature by the generalized vibrational density of states or by a direct integration over the Brillouin zone. Note that presently no interpolation formula is used at low temperatures so thermo_pw can not be used to obtain thermodynamic properties at very low temperatures. The plotted numerical values of the extensive quantities refer to an Avogadro number of unit cells. If you need them per mole you have to divide by the number of formula units in a unit cell.
The input variables that control this option are:

freqmin_input : minimum frequency for phonon dos plot.
                Default: real determined from phonon frequencies
freqmax_input : maximum frequency for phonon dos plot.
                Default: real determined from phonon frequencies
deltafreq     : frequency interval for phonon dos plot.
                Default: real 1 cm^{-1}
ndos_input    : number of frequency points in the dos plot.
                Default: determined from previous data
nq1_d, nq2_d, nq3_d : thick mesh for phonon dos calculation.
                Default: integer 192, 192, 192
phdos_sigma   : the smearing used for phonon dos calculation (in cm^-1).
                Default: real 2. cm^-1
after_disp    : if .TRUE. the dynamical matrices are supposed to be already
                available in files in the current directory. This option
                is needed to restart when the outdir directory has been
                erased and ph.x cannot be run without redoing the scf 
                calculation. The exact restart point depends on the files
                already available on the current directory.
                Default: logical .FALSE.
fildyn        : the name of the dynamical matrix file, as would be 
                specified in the input of ph. 
                To be used when after_disp is .TRUE.. 
                Default: character ' '
zasr          : type of acoustic sum rule applied to the ifc.
                Default: character(len=*) 'Simple'
ltherm_dos    : if .TRUE. the thermal properties are calculated from the 
                phonon dos.
                Default: logical .TRUE.
ltherm_freq   : if .TRUE. the thermal properties are calculated from the
                direct integration using the phonon frequencies.
                Default: logical .TRUE.
flfrc         : file where the interatomic force constants are written
                Default: character(len=*) 'output_frc.dat.g1'
flfrq         : file where matdyn writes the interpolated frequencies
                Default: character(len=*) 'output_frq.dat.g1'
flvec         : file where the eigenvectors of the dynamical matrix are
                written
                Default: character(len=*) 'matdyn.modes'
fldosfrq      : file where the frequencies used to calculate the phonon
                density of states are saved.
                Default: character(len=*) 'save_frequencies.dat'
fldos         : file where the phonon dos is written
                Default: character(len=*) 'output_dos.dat.g1'
fltherm       : file where the harmonic thermodynamic quantities are written
                Default: character(len=*) 'output_therm.dat.g1'
flpsdisp      : postscript file of the phonon dispersions
                Default: character(len=*) 'output_disp'
flpsdos       : postscript file of the phonon dos
                Default: character(len=*) 'output_dos'
flpstherm     : postscript file of the harmonic thermodynamic quantities
                Default: character(len=*) 'output_therm'

This option requires ldisp=.TRUE. in the phonon input.
An example for this option can be found in example04.
Number of tasks for this option: number of parallelizable tasks of the phonon code (smaller but of the order of number of q points times 3N_at, where N_at is the number of atoms in the unit cell).