3.10 what='scf_ph'

With this option the code makes a self-consistent calculation followed by a phonon calculation. The phonon calculation is controlled by the file ph_control and can be at a single q point or on a mesh of q points. The different representations are calculated in parallel when several images are available. No other input variable is necessary. The outputs of this calculation are the dynamical matrices files.
thermo_pw adds to the ph.x code the ability to compute the complex dielectric constant tensor of insulators as a function of a complex frequency for the study of optical properties within time-dependent density functional perturbation theory (TD-DFPT). The code produces also the complex index of refraction for all systems except monoclinic and triclinic. For cubic solids it makes also a plot of the reflectivity for normal incidence and of the adsorption coefficient. As a default the TD-DFPT algorithm uses the Sternheimer equation and a self-consistent loop, but it is also possible to use a Lanczos chain. The option is activated in the ph.x input by setting epsil=.TRUE. and fpol=.TRUE., but at variance with the ph.x code, the frequencies must be specified as complex numbers. The following additional variables can be put in the input of the ph.x code, to select the frequency range and the number of frequencies to compute:

freq_line : if this variable is .TRUE., after the FREQUENCY 
          keyword the code expects the number of frequency 
          points and the starting and final frequencies. 
          If .FALSE. the number of frequencies and a list of 
          frequencies are given. The frequencies are complex 
          numbers and are given with a real and an imaginary 
          part (in Ry), without parenthesis.
          Default: .FALSE.
delta_freq : When freq_line is .TRUE. instead of giving the 
          last frequency of the line one can give the distance 
          between two frequency points delta_freq as a complex 
          number. The last point of the line is calculated using 
          the number of frequencies nfs and the first frequency. 
          When delta_freq is not zero the last frequency is not 
          used and can be omitted.
          Default: complex, (0.0, 0.0).
start_freq : Number of the initial frequency calculated in the 
          job in the sequence of frequencies.
          Default: integer  1
last_freq : Number of the final frequency calculated in the job 
          in the sequence of frequencies.
          Default: integer nfs (total number of frequencies)
lfreq_ev  : If .TRUE. the units of the frequencies are eV 
          instead of the default Ry units.
          Default: logical .FALSE.
linear_im_freq: This option is used only when freq_line=.TRUE. 
          When linear_freq_im is .TRUE., the imaginary part 
          of each frequency is calculated as eta * freq where 
          eta is the imaginary part of the first frequency 
          on the frequency line.
          Default: logical .FALSE.
llanczos: When this flag is .TRUE. at finite frequencies a 
          Lanczos algorithm is used to solve the linear 
          system. Can be very fast but might require much 
          more memory than the standard algorithm. Presently 
          it is incompatible with images.
          Default: .FALSE.
lanczos_steps: steps of the Lanczos chain.
          Default: interger 2000
lanczos_steps_ext:  steps of the extrapolated Lanczos chain
          Default: integer 10000
lanczos_restart_steps: number of steps between saving of the 
          Lanczos status. If 0 the status is saved only at the 
          end of the run. Use recover=.TRUE. to resume an 
          interrupted Lanczos chain or to increase the 
          number of steps.
          Default: integer 0
extrapolation : extrapolation type. Presently only 'no' or 
          'average' are available. In the first case no 
          extrapolation is applied, in the second the average 
          of the beta and gamma is used.
          Default: character 'average'
pseudo_hermitian : when .TRUE. a pseudo-hermitian algorithm is 
          used to make the Lanczos steps. Should be twice 
          faster than the default non hermitian algorithm.
          Default: .TRUE.
only_spectrum : Computes only the spectrum assuming that 
          the Lanczos chain coefficients are in a file. It 
          gives error if the number of requested Lanczos 
          steps is larger than those available on file.
          Default: logical .FALSE.
lcg:      When this flag is .TRUE. a global conjugate 
          gradient algorithm is used to compute the 
          dielectric constant and the phonon frequencies. 
          It will not require mixing, but will use more 
          memory than the standard algorithm (for insulators 
          only). It is not available for the frequency 
          dependent case.
          Default: .FALSE.

When a non zero wave-vector q is specified in the input of the phonon code, the previous options produce the inverse of the dielectric constant as a function of the frequency at the wave-vector q (this option can be used both for insulators and metals).
Additional variables can be specified in the THERMO_PW input to control where the frequency dependent dielectric constant is written and plotted and how the work is divided among images:

flepsilon : beginning of the name of the file where the 
          frequency dependent dielectric constant is 
          written at finite q (the code adds the 
          extensions _re and _im).
          Default: character(len=*) 'epsilon'
flpsepsilon : name of the postscript file where the frequency 
          dependent dielectric constant is plotted at finite q.
          Default: character(len=*) 'output_epsilon'
floptical : beginning of the name of the file where the 
          frequency dependent dielectric constant and the 
          complex index of refraction are written (the code 
          adds the extensions _xx, _yy, and _zz for the solids 
          that need to distinguish the different directions).
          Default: character(len=*) 'optical'
flpsoptical : name of the postscript file where the frequency 
          dependent dielectric constant, the complex index of 
          refraction and for cubic solid also the reflectivity 
          and the absorption coefficient are plotted.
          Default: character(len=*) 'output_optical'
omega_group : number of frequencies calculated together by 
          each image. This variable is used only with images.
          Default: integer 1.

An example for this option can be found in example03, example16, example17, example20, and example21.
Number of tasks for this option: for a phonon calculation the number of parallelizable tasks of the phonon code (smaller but of the order of the number of q points times 3N_at , where N_at is the number of atoms in the unit cell), for a dielectric constant calculation using Sternheimer equation nfs/omega_group, number of frequencies divided by the number of frequencies in each group, for a dielectric constant calculation using Lanczos 1 (images not allowed).

It is also possible to separate the self-consistent and the phonon calculation, by running first thermo_pw.x using what='scf' and then running, on the same directory, thermo_pw.x using what='scf_ph'. The same input can be used in the two calculations, only the thermo_control file need to be changed. The number or processors/pools/images can be changed in the same cases in which this is possible in Quantum ESPRESSO.

Using images in a phonon calculation with the master/slave approach has an overhead because each image must recalculate the initialization and the band structure at each task, or check if the bands are already on disk, calculated previously by the same image. On some systems with slow disks it could be faster to recalculate the bands instead of reading them from disk. It is also possible to use the image breaking suggested by the ph.x code that keeps, as much as possible, on the same image the tasks that require the same initialization without recomputing it. The input variables that control this part of the calculation are:

force_band_calculation : if .TRUE. the bands are never read 
              from disk but recalculated when needed.
              Default: logical .FALSE.
use_ph_images : if .TRUE. each image makes a set of tasks so 
              as to minimize the number of band calculations 
              and phonon initialization.
              Default: logical .FALSE. if nimage>1 .TRUE. 
              nimage=1.
sym_for_diago : When .TRUE. use symmetry to calculate the 
              bands and the unperturbed wavefunctions instead 
              of diagonalizing the Hamiltonian.
              Default: logical .FALSE.