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<input_description distribution="Quantum Espresso" package="NEB" program="neb.x" >
   <toc>
   </toc>
   <intro>
Input data format: { } = optional, [ ] = it depends, | = or

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

BEWARE: TABS, DOS &lt;CR&gt;&lt;LF&gt; 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) :
===============================================================================

&amp;PATH
  ...
/

[ CLIMBING_IMAGES
   list of images, separated by a comma ]
   </intro>
   <namelist name="PATH" >
      <var name="string_method" type="CHARACTER" >
         <default> &apos;neb&apos;
         </default>
         <info>
a string describing the task to be performed:
   &apos;neb&apos;,
   &apos;smd&apos;
         </info>
      </var>
      <var name="restart_mode" type="CHARACTER" >
         <default> &apos;from_scratch&apos;
         </default>
         <info>
&apos;from_scratch&apos;  : from scratch

&apos;restart&apos;       : from previous interrupted run
         </info>
      </var>
      <var name="nstep_path" type="INTEGER" >
         <info>
number of ionic + electronic steps
         </info>
         <default>
1
         </default>
      </var>
      <var name="num_of_images" type="INTEGER" >
         <default> 0
         </default>
         <info>
Number of points used to discretize the path
(it must be larger than 3).
         </info>
      </var>
      <var name="opt_scheme" type="CHARACTER" >
         <default> &apos;quick-min&apos;
         </default>
         <info>
Specify the type of optimization scheme:

&apos;sd&apos;         : steepest descent

&apos;broyden&apos;    : quasi-Newton Broyden&apos;s second method (suggested)

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

&apos;quick-min&apos;  : an optimisation algorithm based on the
               projected velocity Verlet scheme

&apos;langevin&apos;   : finite temperature langevin dynamics of the
               string (smd only). It is used to compute the
               average path and the free-energy profile.
         </info>
      </var>
      <var name="CI_scheme" type="CHARACTER" >
         <default> &apos;no-CI&apos;
         </default>
         <info>
Specify the type of Climbing Image scheme:

&apos;no-CI&apos;      : climbing image is not used

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

&apos;manual&apos;     : images that have to climb are manually selected.
               See also CLIMBING_IMAGES card
         </info>
      </var>
      <var name="first_last_opt" type="LOGICAL" >
         <default> .FALSE.
         </default>
         <info>
Also the first and the last configurations are optimized
&quot;on the fly&quot; (these images do not feel the effect of the springs).
         </info>
      </var>
      <var name="minimum_image" type="LOGICAL" >
         <default> .FALSE.
         </default>
         <info>
Assume a &quot;minimum image criterion&quot; 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.
         </info>
      </var>
      <var name="temp_req" type="REAL" >
         <default> 0.D0 Kelvin
         </default>
         <info>
Temperature used for the langevin dynamics of the string.
         </info>
      </var>
      <var name="ds" type="REAL" >
         <default> 1.D0
         </default>
         <info>
Optimisation step length ( Hartree atomic units ).
If opt_scheme=&quot;broyden&quot;, ds is used as a guess for the
diagonal part of the Jacobian matrix.
         </info>
      </var>
      <vargroup type="REAL" >
         <var name="k_max" >
         </var>
         <var name="k_min" >
         </var>
         <default> 0.1D0 Hartree atomic units
         </default>
         <info>
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.
         </info>
      </vargroup>
      <var name="path_thr" type="REAL" >
         <default> 0.05D0 eV / Angstrom
         </default>
         <info>
The simulation stops when the error ( the norm of the force
orthogonal to the path in eV/A ) is less than path_thr.
         </info>
      </var>
      <var name="use_masses" type="LOGICAL" >
         <default> .FALSE.
         </default>
         <info>
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 &quot;ds&quot;
         </info>
      </var>
      <var name="use_freezing" type="LOGICAL" >
         <default> .FALSE.
         </default>
         <info>
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.
         </info>
      </var>
      <var name="lfcpopt" type="LOGICAL" >
         <see> fcp_mu
         </see>
         <default> .FALSE.
         </default>
         <info>
If .TRUE. perform a constant bias potential (constant-mu)
calculation with ESM method (assume_isolated = &apos;esm&apos; and
esm_bc = &apos;bc2&apos; or &apos;bc3&apos; must be set in SYSTEM namelist).
&apos;fcp_mu&apos; gives the target Fermi energy.
See the header of PW/src/fcp.f90 for documentation
         </info>
      </var>
      <var name="fcp_mu" type="REAL" >
         <see> lfcpopt
         </see>
         <default> 0.d0
         </default>
         <info>
If lfcpopt = .TRUE., gives the target Fermi energy [Ry].
One can specify the total charge of the system for the first
and last image by giving &apos;fcp_tot_charge_first&apos; and &apos;fcp_tot_charge_last&apos;
so that the Fermi energy of these systems will be the target value,
otherwise &apos;first_last_opt&apos; should be .TRUE.
         </info>
      </var>
      <var name="fcp_tot_charge_first" type="REAL" >
         <see> lfcpopt
         </see>
         <default> 0.d0
         </default>
         <info>
Total charge of the system (&apos;tot_charge&apos;) for the first image.
Initial &apos;tot_charge&apos; for intermediate images will be given by
linear interpolation of &apos;fcp_tot_charge_first&apos; and &apos;fcp_tot_charge_last&apos;
         </info>
      </var>
      <var name="fcp_tot_charge_last" type="REAL" >
         <see> lfcpopt
         </see>
         <default> 0.d0
         </default>
         <info>
Total charge of the system (&apos;tot_charge&apos;) for the last image.
Initial &apos;tot_charge&apos; for intermediate images will be given by
linear interpolation of &apos;fcp_tot_charge_first&apos; and &apos;fcp_tot_charge_last&apos;
         </info>
      </var>
   </namelist>
   <card name="CLIMBING_IMAGES" >
      <label>
Optional card, needed only if CI_scheme = &apos;manual&apos;, ignored otherwise !
      </label>
      <syntax>
         <list name="climbing_images_list" type="INTEGER" >
            <format> index1, index2, ... indexN
            </format>
            <info>
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.
            </info>
         </list>
      </syntax>
   </card>
</input_description>
