3.11 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 that are then used to calculate 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 controlled by 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, free energy, 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 extensive quantities plotted in the figures 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
idebye        : if 1, 2, or 3 the code computes the Debye 
                temperature as a function of temperature from 
                the free energy, the vibrational energy, or the 
                heat capacity respectively.
                Default: integer 0
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).