# fix nve/limit command

## Syntax

    fix ID group-ID nve/limit xmax

-   ID, group-ID are documented in [fix](fix) command
-   nve = style name of this fix command
-   xmax = maximum distance an atom can move in one timestep (distance
    units)

## Examples

``` LAMMPS
fix 1 all nve/limit 0.1
```

## Description

Perform constant NVE updates of position and velocity for atoms in the
group each timestep. A limit is imposed on the maximum distance an atom
can move in one timestep. This is useful when starting a simulation with
a configuration containing highly overlapped atoms. Normally this would
generate huge forces which would blow atoms out of the simulation box,
causing LAMMPS to stop with an error.

Using this fix can overcome that problem. Forces on atoms must still be
computable (which typically means 2 atoms must have a separation
distance \> 0.0). But large velocities generated by large forces are
reset to a value that corresponds to a displacement of length *xmax* in
a single timestep. *Xmax* is specified in distance units; see the
[units](units) command for details. The value of *xmax* should be
consistent with the neighbor skin distance and the frequency of neighbor
list re-building, so that pairwise interactions are not missed on
successive timesteps as atoms move. See the [neighbor](neighbor) and
[neigh_modify](neigh_modify) commands for details.

Note that if a velocity reset occurs the integrator will not conserve
energy. On steps where no velocity resets occur, this integrator is
exactly like the [fix nve](fix_nve) command. Since forces are unaltered,
pressures computed by thermodynamic output will still be very large for
overlapped configurations.

:::: note
::: title
Note
:::

You should not use [fix shake](fix_shake) in conjunction with this fix.
That is because fix shake applies constraint forces based on the
predicted positions of atoms after the next timestep. It has no way of
knowing the timestep may change due to this fix, which will cause the
constraint forces to be invalid. A better strategy is to turn off fix
shake when performing initial dynamics that need this fix, then turn fix
shake on when doing normal dynamics with a fixed-size timestep.
::::

## 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.

This fix computes a global scalar which can be accessed by various
[output commands](Howto_output). The scalar is the count of how many
updates of atom\'s velocity/position were limited by the maximum
distance criterion. This should be roughly the number of atoms so
affected, except that updates occur at both the beginning and end of a
timestep in a velocity Verlet timestepping algorithm. This is a
cumulative quantity for the current run, but is re-initialized to zero
each time a run is performed. The scalar value calculated by this fix is
\"extensive\".

No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

> none

## Related commands

[fix nve](fix_nve), [fix nve/noforce](fix_nve_noforce), [pair_style
soft](pair_soft)

## Default

none