# fix imd command

## Syntax

    fix ID group-ID imd trate port keyword values ...

-   ID, group-ID are documented in [fix](fix) command

-   imd = style name of this fix command

-   port = port number on which the fix listens for an IMD client

-   keyword = *unwrap* or *fscale* or *trate* or *nowait*

        *unwrap* arg = *on* or *off*
          off = coordinates are wrapped back into the principal unit cell (default)
          on = "unwrapped" coordinates using the image flags used
        *fscale* arg = factor
          factor = floating point number to scale IMD forces (default: 1.0)
        *trate* arg = transmission rate of coordinate data sets (default: 1)
        *nowait* arg = *on* or *off*
          off = LAMMPS waits to be connected to an IMD client before continuing (default)
          on = LAMMPS listens for an IMD client, but continues with the run

## Examples

``` LAMMPS
fix vmd all imd 5678
fix comm all imd 8888 trate 5 unwrap on fscale 10.0
```

## Description

This fix implements the \"Interactive MD\" (IMD) protocol which allows
realtime visualization and manipulation of MD simulations through the
IMD protocol, as initially implemented in VMD and NAMD. Specifically it
allows LAMMPS to connect an IMD client, for example the [VMD
visualization program](https://www.ks.uiuc.edu/Research/vmd)\_, so that
it can monitor the progress of the simulation and interactively apply
forces to selected atoms.

If LAMMPS is compiled with the pre-processor flag -DLAMMPS_ASYNC_IMD
then fix imd will use POSIX threads to spawn a IMD communication thread
on MPI rank 0 in order to offload data reading and writing from the main
execution thread and potentially lower the inferred latencies for slow
communication links. This feature has only been tested under linux.

The source code for this fix includes code developed by the Theoretical
and Computational Biophysics Group in the Beckman Institute for Advanced
Science and Technology at the University of Illinois at
Urbana-Champaign. We thank them for providing a software interface that
allows codes like LAMMPS to hook to
[VMD](https://www.ks.uiuc.edu/Research/vmd)\_.

Upon initialization of the fix, it will open a communication port on the
node with MPI task 0 and wait for an incoming connection. As soon as an
IMD client is connected, the simulation will continue and the fix will
send the current coordinates of the fix\'s group to the IMD client at
every trate MD step. When using r-RESPA, trate applies to the steps of
the outmost RESPA level. During a run with an active IMD connection also
the IMD client can request to apply forces to selected atoms of the fix
group.

The port number selected must be an available network port number. On
many machines, port numbers \< 1024 are reserved for accounts with
system manager privilege and specific applications. If multiple imd
fixes would be active at the same time, each needs to use a different
port number.

The *nowait* keyword controls the behavior of the fix when no IMD client
is connected. With the default setting of *off*, LAMMPS will wait until
a connection is made before continuing with the execution. Setting
*nowait* to *on* will have the LAMMPS code be ready to connect to a
client, but continue with the simulation. This can for example be used
to monitor the progress of an ongoing calculation without the need to be
permanently connected or having to download a trajectory file.

The *trate* keyword allows to select how often the coordinate data is
sent to the IMD client. It can also be changed on request of the IMD
client through an IMD protocol message. The *unwrap* keyword allows to
send \"unwrapped\" coordinates to the IMD client that undo the wrapping
back of coordinates into the principle unit cell, as done by default in
LAMMPS. The *fscale* keyword allows to apply a scaling factor to forces
transmitted by the IMD client. The IMD protocols stipulates that forces
are transferred in kcal/mol/Angstrom under the assumption that
coordinates are given in Angstrom. For LAMMPS runs with different units
or as a measure to tweak the forces generated by the manipulation of the
IMD client, this option allows to make adjustments.

To connect VMD to a listening LAMMPS simulation on the same machine with
fix imd enabled, one needs to start VMD and load a coordinate or
topology file that matches the fix group. When the VMD command prompts
appears, one types the command line:

    imd connect localhost 5678

This assumes that *fix imd* was started with 5678 as a port number for
the IMD protocol.

The steps to do interactive manipulation of a running simulation in VMD
are the following:

In the Mouse menu of the VMD Main window, select \"Mouse -\> Force -\>
Atom\". You may alternately select \"Residue\", or \"Fragment\" to apply
forces to whole residues or fragments. Your mouse can now be used to
apply forces to your simulation. Click on an atom, residue, or fragment
and drag to apply a force. Click quickly without moving the mouse to
turn the force off. You can also use a variety of 3D position trackers
to apply forces to your simulation. Game controllers or haptic devices
with force-feedback such as the Novint Falcon or Sensable PHANTOM allow
you to feel the resistance due to inertia or interactions with neighbors
that the atoms experience you are trying to move, as if they were real
objects. See the [VMD IMD
Homepage](https://www.ks.uiuc.edu/Research/vmd/imd/)\_ for more details.

If IMD control messages are received, a line of text describing the
message and its effect will be printed to the LAMMPS output screen, if
screen output is active.

## 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 scalar or vector 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. This fix is not invoked
during [energy minimization](minimize).

## Restrictions

This fix is part of the MISC package. It is only enabled if LAMMPS was
built with that package. See the [Build package](Build_package) page for
more info.

When used in combination with VMD, a topology or coordinate file has to
be loaded, which matches (in number and ordering of atoms) the group the
fix is applied to. The fix internally sorts atom IDs by ascending
integer value; in VMD (and thus the IMD protocol) those will be assigned
0-based consecutive index numbers.

When using multiple active IMD connections at the same time, each needs
to use a different port number.

## Related commands

none

## Default

none