# fix neb/spin command

## Syntax

    fix ID group-ID neb/spin Kspring

-   ID, group-ID are documented in [fix](fix) command
-   neb/spin = style name of this fix command

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    Kspring = spring constant for parallel nudging force
    (force/distance units or force units, see parallel keyword)

## Examples

``` LAMMPS
fix 1 active neb/spin 1.0
```

## Description

Add nudging forces to spins in the group for a multi-replica simulation
run via the [neb/spin](neb_spin) command to perform a geodesic nudged
elastic band (GNEB) calculation for finding the transition state.
Hi-level explanations of GNEB are given with the [neb/spin](neb_spin)
command and on the [Howto replica](Howto_replica) doc page. The fix
neb/spin command must be used with the \"neb/spin\" command and defines
how inter-replica nudging forces are computed. A GNEB calculation is
divided in two stages. In the first stage n replicas are relaxed toward
a MEP until convergence. In the second stage, the climbing image scheme
is enabled, so that the replica having the highest energy relaxes toward
the saddle point (i.e. the point of highest energy along the MEP), and a
second relaxation is performed.

The nudging forces are calculated as explained in
[(BessarabB)](BessarabB)). See this reference for more explanation about
their expression.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](fix_modify) options are
relevant to this fix. No global or per-atom quantities are stored by
this fix for access by various [output commands](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command.

The forces due to this fix are imposed during an energy minimization, as
invoked by the [minimize](minimize) command via the [neb/spin](neb_spin)
command.

## Restrictions

This command can only be used if LAMMPS was built with the SPIN package.
See the [Build package](Build_package) doc page for more info.

## Related commands

[neb_spin](neb_spin)

## Default

none

------------------------------------------------------------------------

::: {#BessarabB}
**(BessarabB)** Bessarab, Uzdin, Jonsson, Comp Phys Comm, 196, 335-347
(2015).
:::