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<div class="section" id="fix-addforce-command">
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<span id="index-0"></span><h1>fix addforce command<a class="headerlink" href="#fix-addforce-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="fix-addforce-cuda-command">
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<h1>fix addforce/cuda command<a class="headerlink" href="#fix-addforce-cuda-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 addforce fx fy fz keyword value ...
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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>addforce = style name of this fix command</li>
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<li>fx,fy,fz = force component values (force units)</li>
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</ul>
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<div class="highlight-python"><div class="highlight"><pre>any of fx,fy,fz can be a variable (see below)
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</pre></div>
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</div>
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<ul class="simple">
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<li>zero or more keyword/value pairs may be appended to args</li>
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<li>keyword = <em>every</em> or <em>region</em> or <em>energy</em></li>
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</ul>
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<pre class="literal-block">
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<em>every</em> value = Nevery
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Nevery = add force every this many timesteps
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<em>region</em> value = region-ID
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region-ID = ID of region atoms must be in to have added force
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<em>energy</em> value = v_name
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v_name = variable with name that calculates the potential energy of each atom in the added force field
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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 kick flow addforce 1.0 0.0 0.0
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fix kick flow addforce 1.0 0.0 v_oscillate
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fix ff boundary addforce 0.0 0.0 v_push energy v_espace
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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>Add fx,fy,fz to the corresponding component of force for each atom in
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the group. This command can be used to give an additional push to
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atoms in a simulation, such as for a simulation of Poiseuille flow in
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a channel.</p>
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<p>Any of the 3 quantities defining the force components can be specified
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as an equal-style or atom-style <a class="reference internal" href="variable.html"><em>variable</em></a>, namely <em>fx</em>,
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<em>fy</em>, <em>fz</em>. If the value is a variable, it should be specified as
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v_name, where name is the variable name. In this case, the variable
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will be evaluated each timestep, and its value(s) used to determine
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the force component.</p>
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<p>Equal-style variables can specify formulas with various mathematical
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functions, and include <a class="reference internal" href="thermo_style.html"><em>thermo_style</em></a> command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent force field.</p>
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<p>Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent force
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field with optional time-dependence as well.</p>
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<p>If the <em>every</em> keyword is used, the <em>Nevery</em> setting determines how
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often the forces are applied. The default value is 1, for every
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timestep.</p>
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<p>If the <em>region</em> keyword is used, the atom must also be in the
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specified geometric <a class="reference internal" href="region.html"><em>region</em></a> in order to have force added
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to it.</p>
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<hr class="docutils" />
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<p>Adding a force to atoms implies a change in their potential energy as
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they move due to the applied force field. For dynamics via the “run”
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command, this energy can be optionally added to the system’s potential
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energy for thermodynamic output (see below). For energy minimization
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via the “minimize” command, this energy must be added to the system’s
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potential energy to formulate a self-consistent minimization problem
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(see below).</p>
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<p>The <em>energy</em> keyword is not allowed if the added force is a constant
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vector F = (fx,fy,fz), with all components defined as numeric
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constants and not as variables. This is because LAMMPS can compute
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the energy for each atom directly as E = -x dot F = -(x*fx + y*fy +
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z*fz), so that -Grad(E) = F.</p>
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<p>The <em>energy</em> keyword is optional if the added force is defined with
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one or more variables, and if you are performing dynamics via the
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<a class="reference internal" href="run.html"><em>run</em></a> command. If the keyword is not used, LAMMPS will set
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the energy to 0.0, which is typically fine for dynamics.</p>
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<p>The <em>energy</em> keyword is required if the added force is defined with
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one or more variables, and you are performing energy minimization via
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the “minimize” command. The keyword specifies the name of an
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atom-style <a class="reference internal" href="variable.html"><em>variable</em></a> which is used to compute the
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energy of each atom as function of its position. Like variables used
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for <em>fx</em>, <em>fy</em>, <em>fz</em>, the energy variable is specified as v_name,
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where name is the variable name.</p>
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<p>Note that when the <em>energy</em> keyword is used during an energy
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minimization, you must insure that the formula defined for the
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atom-style <a class="reference internal" href="variable.html"><em>variable</em></a> is consistent with the force
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
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were a spring-like F = kx, then the energy formula should be E =
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-0.5kx^2. If you don’t do this correctly, the minimization will not
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converge properly.</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em> suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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<a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.</p>
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<p>These accelerated styles are part of the USER-CUDA package. They are
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only enabled if 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>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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</div>
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<hr class="docutils" />
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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>.</p>
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<p>The <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> <em>energy</em> option is supported by this
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fix to add the potential “energy” inferred by the added force to the
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system’s potential energy as part of <a class="reference internal" href="thermo_style.html"><em>thermodynamic output</em></a>. This is a fictitious quantity but is
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needed so that the <a class="reference internal" href="minimize.html"><em>minimize</em></a> command can include the
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forces added by this fix in a consistent manner. I.e. there is a
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decrease in potential energy when atoms move in the direction of the
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added force.</p>
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<p>This fix computes a global scalar and a global 3-vector of forces,
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which can be accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. The scalar is the potential
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energy discussed above. The vector is the total force on the group of
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atoms before the forces on individual atoms are changed by the fix.
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The scalar and vector values calculated by this fix are “extensive”.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><em>run</em></a> command.</p>
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<p>The forces due to this fix are imposed during an energy minimization,
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invoked by the <a class="reference internal" href="minimize.html"><em>minimize</em></a> command. You should not
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specify force components with a variable that has time-dependence for
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use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">If you want the fictitious potential energy associated with the
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added forces to be included in the total potential energy of the
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system (the quantity being minimized), you MUST enable the
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<a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> <em>energy</em> option for this fix.</p>
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</div>
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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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<blockquote>
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<div>none</div></blockquote>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="fix_setforce.html"><em>fix setforce</em></a>, <a class="reference internal" href="fix_aveforce.html"><em>fix aveforce</em></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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<p>The option default for the every keyword is every = 1.</p>
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