input_description -distribution {Quantum ESPRESSO} -package PHonon -program ph.x {

    toc {}

    intro {
	@b {Input data format:} { } = optional, [ ] = it depends, # = comment

	@b {Structure of the input data:}
	===============================================================================

	title_line 

	@b &INPUTPH
	   ...
	@b /

	[ xq(1) xq(2) xq(3) ]                        @i {# if @ref ldisp != .true.  and  @ref qplot != .true.}

        [ nqs                                        @i {# if @ref qplot == .true. }
          xq(1,i)    xq(2,i)    xq(3,1)    nq(1)              
          ...
	  xq(1,nqs)  xq(2,nqs)  xq(3,nqs)  nq(nqs) ]     

	[ atom(1)  atom(2)  ... atom(nat_todo) ]     @i {# if @ref nat_todo was specified}
    }

    linecard {
	var title_line -type CHARACTER {
	    info {
		Title of the job, i.e., a line that is reprinted on output.
	    }
	}    
    }

    namelist INPUTPH {

	dimension amass -start 1 -end ntyp -type REAL {
	    default { 0.0 }
	    info {
		Atomic mass [amu] of each atomic type.
		If not specified, masses are read from data file.
	    }
	}    

        var outdir -type CHARACTER {
            default {
                value of the @tt ESPRESSO_TMPDIR environment variable if set;
                @br current directory ('./') otherwise
            }
            info {
                Directory containing input, output, and scratch files; 
                must be the same as specified in the calculation of 
                the unperturbed system.
            }
        }
    
        var prefix -type CHARACTER { 
            default { 'pwscf' }
            info {
                Prepended to input/output filenames; must be the same 
                used in the calculation of unperturbed system.
            }
        }

        var niter_ph -type INTEGER { 
            default { maxter=100 }
            info {
                Maximum number of iterations in a scf step. If you want
                more than 100, edit variable "maxter" in PH/phcom.f90
            }
        }

        var tr2_ph   -type REAL { 
            default { 1e-12 }
            info     { Threshold for self-consistency. }
        }

        var alpha_mix(niter)  -type REAL { 
            default { alpha_mix(1)=0.7 }
            info { 
                Mixing factor (for each iteration) for updating 
                the scf potential:

                vnew(in) = alpha_mix*vold(out) + (1-alpha_mix)*vold(in)
            }
        }

        var nmix_ph   -type INTEGER { 
            default { 4 }
            info {
                Number of iterations used in potential mixing. Using a larger value (8~20)
                can significantly speed up convergence, at the cost of using more memory.
            }
        }

        var verbosity -type CHARACTER { 
            default { 'default' }
            options {
                info { Options are: }
                opt -val {'debug', 'high', 'medium'} { verbose output }
                opt -val {'low', 'default', 'minimal'} { short output }
            }
        }

        var reduce_io -type LOGICAL { 
            default { .false. }
            info {
                Reduce I/O to the strict minimum.

                @b BEWARE: If the input flag @ref reduce_io=.true. was
                used, it is not allowed to restart from an interrupted
                run.
            }
        }

        var max_seconds  -type REAL { 
            default { 1.d7 }
            info { Maximum allowed run time before the job stops smoothly. }
        }

        var dftd3_hess -type CHARACTER { 
            default { 'prefix.hess' }
            info { File where the D3 dispersion hessian matrix is read. }
        }

        var fildyn -type CHARACTER { 
            default { 'matdyn' }
            info { File where the dynamical matrix is written. }
        }

        var fildrho -type CHARACTER { 
            default { ' ' }
            info {
                File where the charge density responses are written. Note that the file
                will actually be saved as @b {${outdir}/_ph0/${prefix}.${fildrho}1}
                where  @b ${outdir}, @b ${prefix} and @b ${fildrho} are the values of the 
                corresponding input variables
            }
        }

        var fildvscf  -type CHARACTER { 
            default { ' ' }
            info {
                File where the the potential variation is written 
                (for later use in electron-phonon calculation, see also fildrho).
            }
        }


        var epsil  -type LOGICAL { 
            default { .false. }
            info {
                If .true. in a q=0 calculation for a non metal the      
                macroscopic dielectric constant of the system is 
                computed. Do not set @ref epsil to .true. if you have a
                metallic system or q/=0: the code will complain and stop.
                
                Note: the input value of @ref epsil will be ignored if @ref ldisp=.true.
                (the code will automatically set @ref epsil to .false. for metals,
                to .true. for insulators: see routine PHonon/PH/prepare_q.f90).
            }
        }

        var lrpa  -type LOGICAL { 
            default { .false. }
            info {
                If .true. the dielectric constant is calculated at the
                RPA level with DV_xc=0.
            }
        }

        var lnoloc  -type LOGICAL { 
            default { .false. }
            info {
                If .true. the dielectric constant is calculated without
                local fields, i.e. by setting DV_H=0 and DV_xc=0. 
            }
        }

        var trans  -type LOGICAL { 
            default { .true. }
            info {
                If .false. the phonons are not computed.
                If @ref trans .and. @ref epsil are both .true.,
		the effective charges are calculated.
		If @ref ldisp is .true., @ref trans=.false. is overridden
                (except for the case of electron-phonon calculations)
            }      
        }
        
        var lraman -type  LOGICAL { 
            default { .false. }
            info { 
                If .true. calculate non-resonant Raman coefficients      
                using second-order response as in:
                M. Lazzeri and F. Mauri, PRL 90, 036401 (2003).
            }
        }

        group {
            label { Optional variables for Raman: }
            
            var eth_rps -type REAL {
                default { 1.0d-9 }
                info { Threshold for calculation of  Pc R |psi>. }
            }
            var eth_ns  -type REAL { 
                default { 1.0e-12 }
                info { Threshold for non-scf wavefunction calculation. } 
            }
            var dek     -type REAL { 
                default { 1.0e-3 }
                info { Delta_xk used for wavefunction derivation wrt k.}
            }
        }

        var recover  -type LOGICAL { 
            default { .false. }
            info { If .true. restart from an interrupted run.}
        }

        var low_directory_check  -type LOGICAL { 
            default { .false. }
            info { If .true. search in the phsave directory only the
                  quantities requested in input.}
        }

        var only_init  -type LOGICAL { 
            default { .false. }
            info {
                If .true. only the bands and other initialization quantities are calculated.
                (used for GRID parallelization)}
        }

        var qplot  -type LOGICAL { 
            default { .false. }
            info { If .true. a list of q points is read from input.}
        }

        var q2d  -type LOGICAL { 
            default { .false. }
            info {
                If .true. three q points and relative weights are
                read from input. The three q points define the rectangle 
                q(:,1) + l (q(:,2)-q(:,1)) + m (q(:,3)-q(:,1)) where
                0< l,m < 1. The weights are integer and those of points two
                and three are the number of points in the two directions.
            }
	}

	var q_in_band_form  -type LOGICAL { 
	    default { .false. }
	    info {
		This flag is used only when qplot is .true. and q2d is
		.false.. When .true. each couple of q points q(:,i+1) and
		q(:,i) define the line from q(:,i) to q(:,i+1) and nq
		points are generated along that line. nq is the weigth of 
		q(:,i). When .false. only the list of q points given as
		input is calculated. The weights are not used.
	    }
	}

	var electron_phonon  -type CHARACTER { 
	    default { ' ' }
	    options {
		info {
		    Options are:
		}
		opt -val 'simple' {
		    Electron-phonon lambda coefficients are computed
		    for a given q and a grid of k-points specified by
		    the variables @ref nk1, @ref nk2, @ref nk3, @ref k1, @ref k2, @ref k3.
		}
		opt -val 'interpolated' {
		    Electron-phonon is calculated by interpolation
		    over the Brillouin Zone as in M. Wierzbowska, et
		    al. arXiv:cond-mat/0504077
		}
                opt -val 'lambda_tetra' {
                    The electron-phonon coefficient \lambda_{q \nu}
		    is calculated with the optimized tetrahedron method.
   	        }
                opt -val 'gamma_tetra' {
                    The phonon linewidth \gamma_{q \nu} is calculated
		    from the electron-phonon interactions
		    using the optimized tetrahedron method.
   	        }
                opt -val 'epa' {
                    Electron-phonon coupling matrix elements are written
                    to file prefix.epa.k for further processing by program
                    epa.x which implements electron-phonon averaged (EPA)
                    approximation as described in G. Samsonidze & B. Kozinsky,
                    Adv. Energy Mater. 2018, 1800246 doi:10.1002/aenm.201800246
                    arXiv:1511.08115
                }
                opt -val 'ahc' {
                    Quantities required for the calculation of phonon-induced
                    electron self-energy are computed and written to the directory
                    @ref ahc_dir. The output files can be read by postahc.x for
                    the calculation of electron self-energy.
                    Available for both metals and insulators.
                    @ref trans=.false. is required.
                }
		info {
		    For metals only, requires gaussian smearing (except for 'ahc').
		
		    If @ref trans=.true., the lambdas are calculated in the same
		    run, using the same k-point grid for phonons and lambdas.
		    If @ref trans=.false., the lambdas are calculated using
		    previously saved DeltaVscf in @ref fildvscf, previously saved
		    dynamical matrix, and the present punch file. This allows
		    the use of a different (larger) k-point grid.
		}
	    }
	}

    var el_ph_nsigma -type INTEGER {
        default { 10 }
        info {
            The number of double-delta smearing values used in an
            electron-phonon coupling calculation.
        }
    }

    var el_ph_sigma -type REAL {
        default { 0.02 }
        info {
            The spacing between double-delta smearing values used in
            an electron-phonon coupling calculation.
        }
    }

    group {
        label { Variables for @ref electron_phonon = 'ahc': }

        var ahc_dir  -type CHARACTER {
            default { outdir // 'ahc_dir/' }
            info {
                Directory where the output binary files are written.
            }
        }
        var ahc_nbnd -type INTEGER {
            status { REQUIRED }
            info {
                Number of bands for which the electron self-energy is to be computed.
            }
        }
        var ahc_nbndskip  -type INTEGER {
            default { 0 }
            info {
                Number of bands to exclude when computing the self-energy. Self-energy
                is computed for bands with indices from @ref ahc_nbndskip+1 to
                @ref ahc_nbndskip+@ref ahc_nbnd. @ref ahc_nbndskip+@ref ahc_nbnd cannot
                exceed nbnd of the preceding SCF or NSCF calculation.
            }
        }
        var skip_upperfan  -type LOGICAL {
            default { .false. }
            info {
                If .true., skip calculation of the upper Fan self-energy, which
                involves solving the Sternheimer equation.
            }
        }
    }

	var lshift_q  -type LOGICAL { 
	    default { .false. }
	    info { 
	    Use a wave-vector grid displaced by half a grid step
            in each direction - meaningful only when ldisp is .true.
            When this option is set, the q2r.x code cannot be used.
	    }
	}

	var zeu  -type LOGICAL { 
	    default { zeu=@ref epsil }
	    info {
		If .true. in a q=0 calculation for a non metal the 
		effective charges are computed from the dielectric
		response. This is the default algorithm. If @ref epsil=.true.
                and @ref zeu=.false. only the dielectric tensor is calculated.
	    }
	}

	var zue  -type LOGICAL { 
	    default { .false. }
	    info {
		If .true. in a q=0 calculation for a non metal the 
		effective charges are computed from the phonon
		density responses. This is an alternative algorithm,
		different from the default one (if @ref trans .and. @ref epsil )
		The results should be the same within numerical noise.
	    }
	}

	var elop -type LOGICAL {
	    default { .false. }
	    info {
		If .true. calculate electro-optic tensor.
	    }
	}

	var fpol  -type LOGICAL { 
	    default { .false. }
	    info {        
                 If .true. calculate dynamic polarizabilities            
                 Requires @ref epsil=.true. ( experimental stage:
                 see example09 for calculation of methane ).
	    }
	}

	var ldisp  -type LOGICAL { 
	    default { .false. }
	    info {
		If .true. the run calculates phonons for a grid of      
		q-points specified by @ref nq1, @ref nq2, @ref nq3 - for direct
		calculation of the entire phonon dispersion.
	    }
	}

	var nogg  -type LOGICAL { 
	    default { .false. }
	    info {
		If .true. disable the "gamma_gamma" trick used to speed
                up calculations at q=0 (phonon wavevector) if the sum over
                the Brillouin Zone includes k=0 only. The gamma_gamma
                trick exploits symmetry and acoustic sum rule to reduce
                the number of linear response calculations to the strict
                minimum, as it is done in code phcg.x. 
	    }
	}

	var asr  -type LOGICAL { 
	    default { .false. }
	    info {
		Apply Acoustic Sum Rule to dynamical matrix, effective charges
                Works only in conjunction with "gamma_gamma" tricks (see above)
	    }
	}

	var ldiag  -type LOGICAL { 
	    default { .false. }
	    info {
		If .true. forces the diagonalization of the dynamical
                matrix also when only a part of the dynamical matrix
                has been calculated. It is used together with @ref start_irr
                and @ref last_irr. If all modes corresponding to a
                given irreducible representation have been calculated,
                the phonon frequencies of that representation are
                correct. The others are zero or wrong. Use with care.
	    }
	}

	var lqdir  -type LOGICAL { 
	    default { .false. }
	    info {
		If .true. ph.x creates inside outdir a separate subdirectory 
                for each q vector. The flag is set to .true. when @ref ldisp=.true.
		and @ref fildvscf /= ' ' or when an electron-phonon 
                calculation is performed. The induced potential is saved 
                separately for each q inside the subdirectories.
	    }
	}

	var search_sym  -type LOGICAL { 
	    default { .true. }
	    info {
                Set it to .false. if you want to disable the mode 
                symmetry analysis. 
	    }
	}

	vargroup -type INTEGER {
	    var nq1
	    var nq2
	    var nq3 
	    default { 0,0,0 }
	    info {
		Parameters of the Monkhorst-Pack grid (no offset) used
		when @ref ldisp=.true. Same meaning as for nk1, nk2, nk3
		in the input of pw.x.
	    }
	}


        vargroup -type INTEGER {
            var nk1
            var nk2
            var nk3
            var k1
            var k2
            var k3
            default { 0,0,0,0,0,0 }
            info {
                When these parameters are specified the phonon program
                runs a pw non-self consistent calculation with a different
                k-point grid thant that used for the charge density.
                This occurs even in the Gamma case.
                @ref nk1, @ref nk2, @ref nk3 are the parameters of the Monkhorst-Pack grid
                with offset determined by @ref k1, @ref k2, @ref k3.
            }
        }

        var diagonalization -type CHARACTER {
            default { 'david' }
            options {
	        info {
                  Diagonalization method for the non-SCF calculations.
 	        }
                opt -val 'david' {
                    Davidson iterative diagonalization with overlap matrix
                    (default). Fast, may in some rare cases fail.
                }

                opt -val 'cg' {
                    Conjugate-gradient-like band-by-band diagonalization.
                    Slower than 'david' but uses less memory and is 
                    (a little bit) more robust.
                }
            }
        }

        var read_dns_bare -type LOGICAL {
            default { .false. }
            info {
                If .true. the PH code tries to read three files in the DFPT+U 
                calculation: dns_orth, dns_bare, d2ns_bare.
                dns_orth and dns_bare are the first-order variations of
                the occupation matrix, while d2ns_bare is the second-order
                variation of the occupation matrix. These matrices are 
                computed only once during the DFPT+U calculation. However,
                their calculation (especially of d2ns_bare) is computationally
                expensive, this is why they are written to file and then can be
                read (e.g. for restart) in order to save time.
            }
        }

        var ldvscf_interpolate -type  LOGICAL {
            default { .false. }
            info {
                If .true., use Fourier interpolation of phonon potential
                to compute the induced part of phonon potential at each
                q point. Results of a dvscf_q2r.x run is needed.
                Requires @ref trans = .false..
            }
        }

        group {
            label { Optional variables for dvscf interpolation: }

            var wpot_dir -type CHARACTER {
                default { outdir // 'w_pot/' }
                info {
                    Directory where the w_pot binary files are written.
                    Must be the same with @ref wpot_dir used in dvscf_q2r.x.
                    The real space potential files are stored in @ref wpot_dir
                    with names ${prefix}.wpot.irc${irc}//"1".
                }
            }
            var do_long_range  -type LOGICAL {
                default { .false. }
                info {
                    If .true., add the long-range part of the potential
                    to the Fourier interpolated potential as in:
                    S. Ponce et al, J. Chem. Phys. 143, 102813 (2015).
                    Reads dielectric matrix and Born effective charges from
                    the ${wpot_dir}/tensors.dat file, written in dvscf_q2r.x.
                    Currently, only the dipole (Frohlich) part is implemented.
                    The quadrupole part is not implemented.
                }
            }
            var do_charge_neutral  -type LOGICAL {
                default { .false. }
                info {
                    If .true., impose charge neutrality on the Born effective
                    charges. Used only if @ref do_long_range = .true..
                }
            }
        }


	group {
	    label { Specification of irreducible representation }

	    var start_irr  -type INTEGER { 
		default { 1 }
		see { last_irr }
		info {      
		    Perform calculations only from @ref start_irr to @ref last_irr 
		    irreducible representations.
		    
		    IMPORTANT:
		       * @ref start_irr must be <= 3*nat
		       * do not specify @ref nat_todo together with 
		         @ref start_irr, @ref last_irr
		}
	    }

	    var last_irr  -type INTEGER { 
		default { 3*nat }
		see { start_irr }
		info {      
		    Perform calculations only from @ref start_irr to @ref last_irr 
		    irreducible representations.
		
		    IMPORTANT:
		       * @ref start_irr must be <= 3*nat
		       * do not specify @ref nat_todo together with 
		         @ref start_irr, @ref last_irr
		}
	    }

	    var nat_todo  -type INTEGER { 
		default { 0, i.e. displace all atoms }
		info {    
		    Choose the subset of atoms to be used in the linear response 
		    calculation: @ref nat_todo atoms, specified in input (see below)
		    are displaced. Can be used to estimate modes for a molecule
		    adsorbed over a surface without performing a full fledged
		    calculation. Use with care, at your own risk, and be aware
		    that this is an approximation and may not work.
		    IMPORTANT:
		       * @ref nat_todo <= nat
                       * if linear-response is calculated for a given atom, it
                         should also be done for all symmetry-equivalent atoms,
			 or else you will get incorrect results
		}
	    }

	    var modenum -type INTEGER { 
		default { 0 }            
		info {
		    For single-mode phonon calculation : modenum is the index of the
		    irreducible representation (irrep) into which the reducible
		    representation formed by the 3*nat atomic displacements are
		    decomposed in order to perform the phonon calculation.
		    Note that a single-mode calculation will not give you the
		    frequency of a single phonon mode: in general, the selected
		    "modenum" is not an eigenvector. What you get on output is
		    a column of the dynamical matrix.
		}
	    }
	}

	group {
	    label { q-point specification } 

	    var start_q  -type INTEGER { 
		default { 1 }
		see { last_q }
		info {      
		    Used only when ldisp=.true..
		    Computes only the q points from @ref start_q to @ref last_q.
		    
		    IMPORTANT:
		       * @ref start_q must be <= @ref nqs (number of q points found)
		       * do not specify @ref nat_todo together with 
		         @ref start_q, @ref last_q
		}
	    }

	    var last_q  -type INTEGER { 
		default { number of q points }
		see { start_q }
		info {      
		    Used only when @ref ldisp=.true..
		    Computes only the q points from @ref start_q to @ref last_q.
		
		    IMPORTANT
		       * @ref last_q must be <= @ref nqs (number of q points)
		       * do not specify @ref nat_todo together with 
		         @ref start_q, @ref last_q
		}
	    }	

	    var dvscf_star  -type STRUCTURE { 
		default { disabled }
		info {      
        It contains the following components:

        @b dvscf_star%open  (logical, default: .false.)
        @b dvscf_star%dir   (character, default: outdir//"Rotated_DVSCF" or the 
                          ESPRESSO_FILDVSCF_DIR environment variable)
        @b dvscf_star%ext   (character, default: "dvscf") the extension to use 
                          for the name of the output files, see below
        @b dvscf_star%basis (character, default: "cartesian") the basis on which 
                          the rotated dvscf will be saved
        @b dvscf_star%pat   (logical, default: false) save an optional file with the
                          displacement patterns and q vector for each dvscf file

        IF dvscf_star%open is .true. use symmetry to compute and store the variation
        of the self-consistent potential on every q* in the star of the present q.

        The rotated dvscf will then be stored in directory dvscf_star%dir with name
        prefix.dvscf_star%ext.q_name//"1". Where q_name is derived from the coordinates
        of the q-point, expressed as fractions in crystalline coordinates
        (notice that ph.x reads q-points in cartesian coordinates).
        E.g. q_cryst= (0, 0.5, -0.25) -> q_name = "0_1o2_-1o4"

        The dvscf can be represented on a basis of cartesian 1-atom displacements
        (dvscf_star%basis='cartesian') or on the basis of the modes at the rotated q-point
        (dvscf_star%basis='modes'). Notice that the el-ph wannier code requires 'cartesian'.
        Each dvscf file comes with a corresponding pattern file with an additional ".pat"
        suffix; this file contains information about the basis and the q-point of the dvscf.

        Note: rotating dvscf can require a large amount of RAM memory and can be i/o
              intensive; in its current implementation all the operations are done
              on a single processor.
        Note2: this feature is currently untested with image parallelisation.
    }
	    }

	    var drho_star  -type STRUCTURE { 
		see {dvscf_star }
		default { disabled }
		info {      
        It contains the following components:

        @b drho_star%open  (logical, default: .false.)
        @b drho_star%dir   (character, default: outdir//"Rotated_DRHO" or the 
                         ESPRESSO_FILDRHO_DIR environment variable)
        @b drho_star%ext   (character, default: "drho") the extension to use
                         for the name of the output files, see below
        @b drho_star%basis (character, default: "modes") the basis on which
                         the rotated drho will be saved
        @b drho_star%pat   (logical, default: true) save an optional file with the
                         displacement patterns and q vector for each drho file

        Like @ref dvscf_star, but for the perturbation of the charge density.
        Notice that the defaults are different.
    }
	    } 	    
	}   
    }


    choose {
	when -test "ldisp != .true.   and   qplot != .true." {

	    linecard {
		list xq_list -type REAL {
		    format { xq(1)  xq(2)  xq(3) }
		    info {
			The phonon wavevector, in units of 2pi/a0
			(a0 = lattice parameter).
			Not used if @ref ldisp=.true. or @ref qplot=.true.
		    }
		}
	    }
	} 
	elsewhen -test "qplot == .true." {

	    label { Specification of q points when @ref qplot == .true. }

	    card qPointsSpecs -nameless 1 {
		syntax {
		    line {
			var nqs -type INTEGER {
			    info { Number of q points in the list. Used only if @ref qplot=.true.
			    }
			}
		    }
		    
		    table qPoints {
			rows -start 1 -end nqs {
			    colgroup -type REAL {
				info {
                                    q-point coordinates; used only with @ref ldisp=.true. and qplot=.true.
				    The phonon wavevector, in units of 2pi/a0 (a0 = lattice parameter). 
				    The meaning of these q points and their weights nq depend on the 
				    flags q2d and q_in_band_form. (NB: nq is integer)
				}
				col xq1
				col xq2
				col xq3
			    }
			    col nq -type INTEGER {
				info { 
				    The weight of the q-point; the meaning of nq depends 
				    on the flags q2d and q_in_band_form. }
			    }
			}
		    }
		}
	    }
	}
    }

    choose {
        when -test "nat_todo was specified" {
	    linecard {
		list nat_todo_list -type INTEGER {
		    format { atom(1)  atom(2) ... atom(nat_todo) }
		    info {
			Contains the list of indices of atoms used in the
			calculation if @ref nat_todo is specified.
		    }
		}
	    }
	}
    }

    section -title { ADDITIONAL INFORMATION } {

	text {

NB: The program ph.x writes on the tmp_dir/_ph0/{prefix}.phsave directory
a file for each representation of each q point. This file is called
dynmat.#iq.#irr.xml where #iq is the number of the q point and #irr
is the number of the representation. These files contain the
contribution to the dynamical matrix of the irr representation for the
iq point. 

If @ref recover=.true. ph.x does not recalculate the
representations already saved in the tmp_dir/_ph0/{prefix}.phsave
directory.  Moreover ph.x writes on the files patterns.#iq.xml in the
tmp_dir/_ph0/{prefix}.phsave directory the displacement patterns that it
is using. If @ref recover=.true. ph.x does not recalculate the
displacement patterns found in the tmp_dir/_ph0/{prefix}.phsave directory.

This mechanism allows:

  1) To recover part of the ph.x calculation even if the recover file
     or files are corrupted. You just remove the _ph0/{prefix}.recover
     files from the tmp_dir directory. You can also remove all the _ph0
     files and keep only the _ph0/{prefix}.phsave directory.

  2) To split a phonon calculation into several jobs for different
     machines (or set of nodes). Each machine calculates a subset of
     the representations and saves its dynmat.#iq.#irr.xml files on
     its tmp_dir/_ph0/{prefix}.phsave directory. Then you collect all the
     dynmat.#iq.#irr.xml files in one directory and run ph.x to
     collect all the dynamical matrices and diagonalize them.

NB: To split the q points in different machines, use the input
variables start_q and last_q. To split the irreducible
representations, use the input variables @ref start_irr, @ref last_irr. Please
note that different machines will use, in general, different
displacement patterns and it is not possible to recollect partial
dynamical matrices generated with different displacement patterns.  A
calculation split into different machines will run as follows: A
preparatory run of ph.x with @ref start_irr=0, @ref last_irr=0 produces the sets
of displacement patterns and save them on the patterns.#iq.xml files.
These files are copied in all the tmp_dir/_ph0/{prefix}.phsave directories
of the machines where you plan to run ph.x. ph.x is run in different
machines with complementary sets of start_q, last_q, @ref start_irr and
@ref last_irr variables.  All the files dynmat.#iq.#irr.xml are
collected on a single tmp_dir/_ph0/{prefix}.phsave directory (remember to
collect also dynmat.#iq.0.xml).  A final run of ph.x in this
machine collects all the data contained in the files and diagonalizes
the dynamical matrices.  This is done requesting a complete dispersion
calculation without using start_q, last_q, @ref start_irr, or @ref last_irr.
See an example in examples/GRID_example.

On parallel machines the q point and the irreps calculations can be split 
automatically using the -nimage flag. See the phonon user guide for further
information.

}  
}  
}
