Input File Description

Program: neb.x / NEB / Quantum Espresso

TABLE OF CONTENTS

INTRODUCTION

&PATH

string_method | restart_mode | nstep_path | num_of_images | opt_scheme | CI_scheme | first_last_opt | temp_req | ds | k_max | k_min | path_thr | use_masses | use_freezing

CLIMBING_IMAGES

index1, index2, ... indexN

INTRODUCTION

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

string_method CHARACTER
Default: 'neb'
a string describing the task to be performed:
   'neb',
   'smd'
         
restart_mode CHARACTER
Default: 'from_scratch'
'from_scratch'  : from scratch

'restart'       : from previous interrupted run
         
nstep_path INTEGER
Default: 1
number of ionic + electronic steps
         
num_of_images INTEGER
Default: 0
Number of points used to discretize the path
(it must be larger than 3).
         
opt_scheme CHARACTER
Default: 'quick-min'
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.
         
CI_scheme CHARACTER
Default: 'no-CI'
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
         
first_last_opt LOGICAL
Default: .FALSE.
Also the first and the last configurations are optimized
"on the fly" (these images do not feel the effect of the springs).
         
temp_req REAL
Default: 0.D0 Kelvin
Temperature used for the langevin dynamics of the string.
         
ds REAL
Default: 1.D0
Optimisation step length ( Hartree atomic units ).
If opt_scheme="broyden", ds is used as a guess for the
diagonal part of the Jacobian matrix.
         
k_max, k_min REAL
Default: 0.1D0 Hartree atomic units
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.
         
path_thr REAL
Default: 0.05D0 eV / Angstrom
The simulation stops when the error ( the norm of the force
orthogonal to the path in eV/A ) is less than path_thr.
         
use_masses LOGICAL
Default: .FALSE.
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"
         
use_freezing LOGICAL
Default: .FALSE.
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.
         

Card: CLIMBING_IMAGES

Optional card, needed only if CI_scheme = 'manual', ignored otherwise !

Syntax:

CLIMBING_IMAGES
index1, index2, ... indexN   

Description of items:


index1, index2, ... indexN INTEGER
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.
            
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