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<div class="section" id="fix-gravity-command">
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<span id="index-0"></span><h1>fix gravity command<a class="headerlink" href="#fix-gravity-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="fix-gravity-cuda-command">
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<h1>fix gravity/cuda command<a class="headerlink" href="#fix-gravity-cuda-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="fix-gravity-omp-command">
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<h1>fix gravity/omp command<a class="headerlink" href="#fix-gravity-omp-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 gravity magnitude style args
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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 are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>gravity = style name of this fix command</li>
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<li>magnitude = size of acceleration (force/mass units)</li>
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<li>magnitude can be a variable (see below)</li>
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<li>style = <em>chute</em> or <em>spherical</em> or <em>gradient</em> or <em>vector</em></li>
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</ul>
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<pre class="literal-block">
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<em>chute</em> args = angle
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angle = angle in +x away from -z or -y axis in 3d/2d (in degrees)
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angle can be a variable (see below)
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<em>spherical</em> args = phi theta
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phi = azimuthal angle from +x axis (in degrees)
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theta = angle from +z or +y axis in 3d/2d (in degrees)
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phi or theta can be a variable (see below)
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<em>vector</em> args = x y z
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x y z = vector direction to apply the acceleration
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x or y or z can be a variable (see below)
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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 1 all gravity 1.0 chute 24.0
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fix 1 all gravity v_increase chute 24.0
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fix 1 all gravity 1.0 spherical 0.0 -180.0
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fix 1 all gravity 10.0 spherical v_phi v_theta
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fix 1 all gravity 100.0 vector 1 1 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>Impose an additional acceleration on each particle in the group. This
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fix is typically used with granular systems to include a “gravity”
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term acting on the macroscopic particles. More generally, it can
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represent any kind of driving field, e.g. a pressure gradient inducing
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a Poiseuille flow in a fluid. Note that this fix operates differently
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than the <a class="reference internal" href="fix_addforce.html"><em>fix addforce</em></a> command. The addforce fix
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adds the same force to each atom, independent of its mass. This
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command imparts the same acceleration to each atom (force/mass).</p>
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<p>The <em>magnitude</em> of the acceleration is specified in force/mass units.
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For granular systems (LJ units) this is typically 1.0. See the
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<a class="reference internal" href="units.html"><em>units</em></a> command for details.</p>
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<p>Style <em>chute</em> is typically used for simulations of chute flow where
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the specified <em>angle</em> is the chute angle, with flow occurring in the +x
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direction. For 3d systems, the tilt is away from the z axis; for 2d
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systems, the tilt is away from the y axis.</p>
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<p>Style <em>spherical</em> allows an arbitrary 3d direction to be specified for
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the acceleration vector. <em>Phi</em> and <em>theta</em> are defined in the usual
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spherical coordinates. Thus for acceleration acting in the -z
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direction, <em>theta</em> would be 180.0 (or -180.0). <em>Theta</em> = 90.0 and
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<em>phi</em> = -90.0 would mean acceleration acts in the -y direction. For
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2d systems, <em>phi</em> is ignored and <em>theta</em> is an angle in the xy plane
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where <em>theta</em> = 0.0 is the y-axis.</p>
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<p>Style <em>vector</em> imposes an acceleration in the vector direction given
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by (x,y,z). Only the direction of the vector is important; it’s
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length is ignored. For 2d systems, the <em>z</em> component is ignored.</p>
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<p>Any of the quantities <em>magnitude</em>, <em>angle</em>, <em>phi</em>, <em>theta</em>, <em>x</em>, <em>y</em>,
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<em>z</em> which define the gravitational magnitude and direction, can be
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specified as an equal-style <a class="reference internal" href="variable.html"><em>variable</em></a>. If the value is
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a variable, it should be specified as v_name, where name is the
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variable name. In this case, the variable will be evaluated each
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timestep, and its value used to determine the quantity. You should
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insure that the variable calculates a result in the approriate units,
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e.g. force/mass or degrees.</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 gravitational
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field.</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em>, <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.</p>
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<p>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. 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 gravitational potential energy of the system 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>.</p>
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<p>This fix computes a global scalar which can be accessed by various
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<a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. This scalar is the
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gravitational potential energy of the particles in the defined field,
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namely mass * (g dot x) for each particles, where x and mass are the
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particles position and mass, and g is the gravitational field. The
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scalar value calculated by this fix is “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. 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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<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="atom_style.html"><em>atom_style sphere</em></a>, <a class="reference internal" href="fix_addforce.html"><em>fix addforce</em></a></p>
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<p><strong>Default:</strong> none</p>
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