    ------------------------------------------------------------------------
    INPUT FILE DESCRIPTION

    Program: neb.x / NEB / Quantum Espresso
    ------------------------------------------------------------------------

    Input data format: { } = optional, [ ] = it depends, | = or

    All quantities whose dimensions are not explicitly specified are in
    RYDBERG ATOMIC UNITS

    BEWARE: TABS, DOS <CR><LF> CHARACTERS ARE POTENTIAL SOURCES OF TROUBLE

    General input file structure:
    ===============================================================================

    neb.x DOES NOT READ FROM STANDARD INPUT
    There are two ways for running a calculation with neb.x:
    1) specifying a file to parse with the ./neb.x -inp or
     neb.x -input command line option.
    2) or specifying the number of copies of PWscf input ./neb.x -input\_images.

    For case 1) a file containing KEYWORDS has to be written (see below).
    These KEYWORDS tells the parser which part of the file regards neb specifics
    and which part regards the energy/force engine (at the moment only PW).
    After the parsing different files are generated: neb.dat, with
    neb specific variables and a set of pw_*.in PWscf input files like
    one for each input position. All options for a single SCF calculation apply.

    The general structure of the file to be parsed is:

    BEGIN
    BEGIN_PATH_INPUT
    ... neb specific namelists and cards
    END_PATH_INPUT
    BEGIN_ENGINE_INPUT
    ...pw specific namelists and cards
    BEGIN_POSITIONS
    FIRST_IMAGE
    ...pw ATOMIC_POSITIONS card
    INTERMEDIATE_IMAGE
    ...pw ATOMIC_POSITIONS card
    LAST_IMAGE
    ...pw ATOMIC_POSITIONS card
    END_POSITIONS
    ... other pw specific cards
    END_ENGINE_INPUT
    END

    For case 2) neb.dat and all pw_1.in, pw_2.in ... should be already present.

    Structure of the input data (file neb.dat) :
    ===============================================================================

    &PATH
      ...
    /

    [ CLIMBING_IMAGES
       list of images, separated by a comma ]

    ========================================================================
    NAMELIST: &PATH

       +--------------------------------------------------------------------
       Variable:       string_method

       Type:           CHARACTER
       Default:        'neb'
       Description:    a string describing the task to be performed:
                          'neb',
                          'smd'
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       restart_mode

       Type:           CHARACTER
       Default:        'from_scratch'
       Description:    'from_scratch'  : from scratch

                       'restart'       : from previous interrupted run
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       nstep_path

       Type:           INTEGER
       Description:    number of ionic + electronic steps
       Default:        1
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       num_of_images

       Type:           INTEGER
       Default:        0
       Description:    Number of points used to discretize the path
                       (it must be larger than 3).
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       opt_scheme

       Type:           CHARACTER
       Default:        'quick-min'
       Description:    Specify the type of optimization scheme:

                       'sd'         : steepest descent

                       'broyden'    : quasi-Newton Broyden's second method (suggested)

                       'broyden2'   : another variant of the quasi-Newton Broyden's
                                      second method to be tested and compared with the
                                      previous one.

                       'quick-min'  : an optimisation algorithm based on the
                                      projected velocity Verlet scheme

                       'langevin'   : finite temperature langevin dynamics of the
                                      string (smd only). It is used to compute the
                                      average path and the free-energy profile.
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       CI_scheme

       Type:           CHARACTER
       Default:        'no-CI'
       Description:    Specify the type of Climbing Image scheme:

                       'no-CI'      : climbing image is not used

                       'auto'       : original CI scheme. The image highest in energy
                                      does not feel the effect of springs and is
                                      allowed to climb along the path

                       'manual'     : images that have to climb are manually selected.
                                      See also CLIMBING_IMAGES card
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       first_last_opt

       Type:           LOGICAL
       Default:        .FALSE.
       Description:    Also the first and the last configurations are optimized
                       "on the fly" (these images do not feel the effect of the springs).
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       minimum_image

       Type:           LOGICAL
       Default:        .FALSE.
       Description:    Assume a "minimum image criterion" to build the path. If an atom
                       moves by more than half the length of a crystal axis between one
                       image and the next in the input (before interpolation),
                       an appropriate periodic replica of that atom is chosen.
                       Useful to avoid jumps in the initial reaction path.
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       temp_req

       Type:           REAL
       Default:        0.D0 Kelvin
       Description:    Temperature used for the langevin dynamics of the string.
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       ds

       Type:           REAL
       Default:        1.D0
       Description:    Optimisation step length ( Hartree atomic units ).
                       If opt_scheme="broyden", ds is used as a guess for the
                       diagonal part of the Jacobian matrix.
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variables:      k_max, k_min

       Type:           REAL
       Default:        0.1D0 Hartree atomic units
       Description:    Set them to use a Variable Elastic Constants scheme
                       elastic constants are in the range [ k_min, k_max ]
                       this is useful to rise the resolution around the saddle point.
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       path_thr

       Type:           REAL
       Default:        0.05D0 eV / Angstrom
       Description:    The simulation stops when the error ( the norm of the force
                       orthogonal to the path in eV/A ) is less than path_thr.
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       use_masses

       Type:           LOGICAL
       Default:        .FALSE.
       Description:    If. TRUE. the optimisation of the path is performed using
                       mass-weighted coordinates. Useful together with quick-min
                       optimization scheme, if some bonds are much stiffer than
                       others. By assigning a larger (fictitious) mass to atoms
                       with stiff bonds, one may use a longer time step "ds"
       +--------------------------------------------------------------------

       +--------------------------------------------------------------------
       Variable:       use_freezing

       Type:           LOGICAL
       Default:        .FALSE.
       Description:    If. TRUE. the images are optimised according to their error:
                       only those images with an error larger than half of the largest
                       are optimised. The other images are kept frozen.
       +--------------------------------------------------------------------

    ===END OF NAMELIST======================================================

    ========================================================================
    CARD: CLIMBING_IMAGES

       OPTIONAL CARD, NEEDED ONLY IF CI_SCHEME = 'MANUAL', IGNORED OTHERWISE !

       /////////////////////////////////////////
       // Syntax:                             //
       /////////////////////////////////////////

          CLIMBING_IMAGES
             index1, index2, ... indexN

       /////////////////////////////////////////

       DESCRIPTION OF ITEMS:

          +--------------------------------------------------------------------
          Variables:      index1, index2, ... indexN

          Type:           INTEGER
          Description:    index1, index2, ..., indexN are indices of the images to which the
                          Climbing-Image procedure apply. If more than one image is specified
                          they must be separated by a comma.
          +--------------------------------------------------------------------

    ===END OF CARD==========================================================