# fix spring/self command

## Syntax

    fix ID group-ID spring/self K dir

-   ID, group-ID are documented in [fix](fix) command
-   spring/self = style name of this fix command
-   K = spring constant (force/distance units)
-   dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz)

## Examples

``` LAMMPS
fix tether boundary-atoms spring/self 10.0
fix zrest  move spring/self 10.0 z
```

## Description

Apply a spring force independently to each atom in the group to tether
it to its initial position. The initial position for each atom is its
location at the time the fix command was issued. At each timestep, the
magnitude of the force on each atom is -Kr, where r is the displacement
of the atom from its current position to its initial position. The
distance r correctly takes into account any crossings of periodic
boundary by the atom since it was in its initial position.

With the (optional) dir flag, one can select in which direction the
spring force is applied. By default, the restraint is applied in all
directions, but it can be limited to the xy-, xz-, yz-plane and the x-,
y-, or z-direction, thus restraining the atoms to a line or a plane,
respectively.

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

This fix writes the original coordinates of tethered atoms to [binary
restart files](restart), so that the spring effect will be the same in a
restarted simulation. See the [read_restart](read_restart) command for
info on how to re-specify a fix in an input script that reads a restart
file, so that the operation of the fix continues in an uninterrupted
fashion.

The [fix_modify](fix_modify) *energy* option is supported by this fix to
add the energy stored in the per-atom springs to the global potential
energy of the system as part of [thermodynamic output](thermo_style).
The default setting for this fix is [fix_modify energy no](fix_modify).

The [fix_modify](fix_modify) *respa* option is supported by this fix.
This allows to set at which level of the [r-RESPA](run_style) integrator
the fix is adding its forces. Default is the outermost level.

This fix computes a global scalar which can be accessed by various
[output commands](Howto_output). The scalar is an energy which is the
sum of the spring energy for each atom, where the per-atom energy is 0.5
\* K \* r\^2. 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.

The forces due to this fix are imposed during an energy minimization,
invoked by the [minimize](minimize) command.

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

If you want the per-atom spring energy to be included in the total
potential energy of the system (the quantity being minimized), you MUST
enable the [fix_modify](fix_modify) *energy* option for this fix.
::::

## Restrictions

> none

## Related commands

[fix drag](fix_drag), [fix spring](fix_spring), [fix smd](fix_smd), [fix
spring/rg](fix_spring_rg)

## Default

none