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<div class="section" id="fix-imd-command">
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<span id="index-0"></span><h1>fix imd command<a class="headerlink" href="#fix-imd-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID imd trate port keyword values ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>imd = style name of this fix command</li>
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<li>port = port number on which the fix listens for an IMD client</li>
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<li>keyword = <em>unwrap</em> or <em>fscale</em> or <em>trate</em></li>
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</ul>
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<pre class="literal-block">
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<em>unwrap</em> arg = <em>on</em> or <em>off</em>
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off = coordinates are wrapped back into the principal unit cell (default)
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on = "unwrapped" coordinates using the image flags used
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<em>fscale</em> arg = factor
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factor = floating point number to scale IMD forces (default: 1.0)
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<em>trate</em> arg = transmission rate of coordinate data sets (default: 1)
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<em>nowait</em> arg = <em>on</em> or <em>off</em>
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off = LAMMPS waits to be connected to an IMD client before continuing (default)
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on = LAMMPS listens for an IMD client, but continues with the run
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix vmd all imd 5678
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fix comm all imd 8888 trate 5 unwrap on fscale 10.0
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>This fix implements the “Interactive MD” (IMD) protocol which allows
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realtime visualization and manipulation of MD simulations through the
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IMD protocol, as initially implemented in VMD and NAMD. Specifically
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it allows LAMMPS to connect an IMD client, for example the <a class="reference external" href="http://www.ks.uiuc.edu/Research/vmd">VMD visualization program</a>, so that it can monitor the progress of the
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simulation and interactively apply forces to selected atoms.</p>
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<p>If LAMMPS is compiled with the preprocessor flag -DLAMMPS_ASYNC_IMD
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then fix imd will use POSIX threads to spawn a IMD communication
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thread on MPI rank 0 in order to offload data reading and writing
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from the main execution thread and potentially lower the inferred
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latencies for slow communication links. This feature has only been
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tested under linux.</p>
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<p>There are example scripts for using this package with LAMMPS in
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examples/USER/imd. Additional examples and a driver for use with the
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Novint Falcon game controller as haptic device can be found at:
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<a class="reference external" href="http://sites.google.com/site/akohlmey/software/vrpn-icms">http://sites.google.com/site/akohlmey/software/vrpn-icms</a>.</p>
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<p>The source code for this fix includes code developed by the
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Theoretical and Computational Biophysics Group in the Beckman
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Institute for Advanced Science and Technology at the University of
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Illinois at Urbana-Champaign. We thank them for providing a software
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interface that allows codes like LAMMPS to hook to <a class="reference external" href="http://www.ks.uiuc.edu/Research/vmd">VMD</a>.</p>
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<p>Upon initialization of the fix, it will open a communication port on
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the node with MPI task 0 and wait for an incoming connection. As soon
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as an IMD client is connected, the simulation will continue and the
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fix will send the current coordinates of the fix’s group to the IMD
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client at every trate MD step. When using r-RESPA, trate applies to
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the steps of the outmost RESPA level. During a run with an active IMD
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connection also the IMD client can request to apply forces to selected
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atoms of the fix group.</p>
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<p>The port number selected must be an available network port number. On
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many machines, port numbers < 1024 are reserved for accounts with
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system manager privilege and specific applications. If multiple imd
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fixes would be active at the same time, each needs to use a different
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port number.</p>
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<p>The <em>nowait</em> keyword controls the behavior of the fix when no IMD
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client is connected. With the default setting of <em>off</em>, LAMMPS will
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wait until a connection is made before continuing with the
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execution. Setting <em>nowait</em> to <em>on</em> will have the LAMMPS code be ready
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to connect to a client, but continue with the simulation. This can for
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example be used to monitor the progress of an ongoing calculation
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without the need to be permanently connected or having to download a
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trajectory file.</p>
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<p>The <em>trate</em> keyword allows to select how often the coordinate data is
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sent to the IMD client. It can also be changed on request of the IMD
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client through an IMD protocol message. The <em>unwrap</em> keyword allows
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to send “unwrapped” coordinates to the IMD client that undo the
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wrapping back of coordinates into the principle unit cell, as done by
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default in LAMMPS. The <em>fscale</em> keyword allows to apply a scaling
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factor to forces transmitted by the IMD client. The IMD protocols
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stipulates that forces are transferred in kcal/mol/angstrom under the
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assumption that coordinates are given in angstrom. For LAMMPS runs
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with different units or as a measure to tweak the forces generated by
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the manipulation of the IMD client, this option allows to make
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adjustments.</p>
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<p>To connect VMD to a listening LAMMPS simulation on the same machine
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with fix imd enabled, one needs to start VMD and load a coordinate or
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topology file that matches the fix group. When the VMD command
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prompts appears, one types the command line:</p>
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<div class="highlight-python"><div class="highlight"><pre>imd connect localhost 5678
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</pre></div>
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</div>
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<p>This assumes that <em>fix imd</em> was started with 5678 as a port
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number for the IMD protocol.</p>
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<p>The steps to do interactive manipulation of a running simulation in
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VMD are the following:</p>
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<p>In the Mouse menu of the VMD Main window, select “Mouse -> Force ->
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Atom”. You may alternately select “Residue”, or “Fragment” to apply
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forces to whole residues or fragments. Your mouse can now be used to
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apply forces to your simulation. Click on an atom, residue, or
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fragment and drag to apply a force. Click quickly without moving the
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mouse to turn the force off. You can also use a variety of 3D position
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trackers to apply forces to your simulation. Game controllers or haptic
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devices with force-feedback such as the Novint Falcon or Sensable
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PHANTOM allow you to feel the resistance due to inertia or interactions
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with neighbors that the atoms experience you are trying to move, as if
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they were real objects. See the <a class="reference external" href="http://www.ks.uiuc.edu/Research/vmd/imd/">VMD IMD Homepage</a> and the
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<a class="reference external" href="http://sites.google.com/site/akohlmey/software/vrpn-icms">VRPN-ICMS Homepage</a> for more details.</p>
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<p>If IMD control messages are received, a line of text describing the
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message and its effect will be printed to the LAMMPS output screen, if
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screen output is active.</p>
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</div>
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>. None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options
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are relevant to this fix. No global scalar or vector or per-atom
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quantities are stored by this fix for access by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. No parameter of this fix can
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be used with the <em>start/stop</em> keywords of the <a class="reference internal" href="run.html"><em>run</em></a> command.
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This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>This fix is part of the USER-MISC package. It is only enabled if
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LAMMPS was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>When used in combination with VMD, a topology or coordinate file has
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to be loaded, which matches (in number and ordering of atoms) the
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group the fix is applied to. The fix internally sorts atom IDs by
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ascending integer value; in VMD (and thus the IMD protocol) those will
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be assigned 0-based consecutive index numbers.</p>
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<p>When using multiple active IMD connections at the same time, each
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needs to use a different port number.</p>
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<p><strong>Related commands:</strong> none</p>
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<p><strong>Default:</strong> none</p>
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