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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<div class="section" id="fix-efield-command">
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<span id="index-0"></span><h1>fix efield command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">efield</span> <span class="n">ex</span> <span class="n">ey</span> <span class="n">ez</span> <span class="n">keyword</span> <span class="n">value</span> <span class="o">...</span>
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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"><span class="doc">fix</span></a> command</li>
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<li>efield = style name of this fix command</li>
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<li>ex,ey,ez = E-field component values (electric field units)</li>
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<li>any of ex,ey,ez can be a variable (see below)</li>
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<li>zero or more keyword/value pairs may be appended to args</li>
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<li>keyword = <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>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 E-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</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">fix</span> <span class="n">kick</span> <span class="n">external</span><span class="o">-</span><span class="n">field</span> <span class="n">efield</span> <span class="mf">1.0</span> <span class="mf">0.0</span> <span class="mf">0.0</span>
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<span class="n">fix</span> <span class="n">kick</span> <span class="n">external</span><span class="o">-</span><span class="n">field</span> <span class="n">efield</span> <span class="mf">0.0</span> <span class="mf">0.0</span> <span class="n">v_oscillate</span>
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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</h2>
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<p>Add a force F = qE to each charged atom in the group due to an
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external electric field being applied to the system. If the system
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contains point-dipoles, also add a torque on the dipoles due to the
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external electric field.</p>
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<p>For charges, any of the 3 quantities defining the E-field components
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can be specified as an equal-style or atom-style
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<a class="reference internal" href="variable.html"><span class="doc">variable</span></a>, namely <em>ex</em>, <em>ey</em>, <em>ez</em>. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value used to determine the E-field component.</p>
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<p>For point-dipoles, equal-style variables can be used, but atom-style
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variables are not currently supported, since they imply a spatial
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gradient in the electric field which means additional terms with
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gradients of the field are required for the force and torque on
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dipoles.</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"><span class="doc">thermo_style</span></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 E-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 E-field
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with optional time-dependence as well.</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"><span class="doc">region</span></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 or torque to atoms implies a change in their potential
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energy as they move or rotate due to the applied E-field.</p>
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<p>For dynamics via the “run” command, this energy can be optionally
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added to the system’s potential energy for thermodynamic output (see
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below). For energy minimization via the “minimize” command, this
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energy must be added to the system’s potential energy to formulate a
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self-consistent minimization problem (see below).</p>
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<p>The <em>energy</em> keyword is not allowed if the added field is a constant
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vector (ex,ey,ez), with all components defined as numeric constants
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and not as variables. This is because LAMMPS can compute the energy
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for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey
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+ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles
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the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey +
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muz*ez).</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"><span class="doc">run</span></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 for charged particles. It is not required for
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point-dipoles, but a warning is issued since the minimizer in LAMMPS
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does not rotate dipoles, so you should not expect to be able to
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minimize the orientation of dipoles in an applied electric field.</p>
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<p>The <em>energy</em> keyword specifies the name of an atom-style
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<a class="reference internal" href="variable.html"><span class="doc">variable</span></a> which is used to compute the energy of each
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atom as function of its position. Like variables used for <em>ex</em>, <em>ey</em>,
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<em>ez</em>, the energy variable is specified as v_name, where name is the
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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"><span class="doc">variable</span></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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due to the electric field were a spring-like F = kx, then the energy
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formula should be E = -0.5kx^2. If you don’t do this correctly, the
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minimization will not converge properly.</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</h2>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>.</p>
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<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option is supported by this
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fix to add the potential “energy” inferred by the added force due to
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the electric field to the system’s potential energy as part of
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<a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>. This is a fictitious
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quantity but is needed so that the <a class="reference internal" href="minimize.html"><span class="doc">minimize</span></a> command
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can include the forces added by this fix in a consistent manner.
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I.e. there is a decrease in potential energy when atoms move in the
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direction of the added force due to the electric field.</p>
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<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>respa</em> option is supported by this
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fix. This allows to set at which level of the <a class="reference internal" href="run_style.html"><span class="doc">r-RESPA</span></a>
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integrator the fix adding its forces. Default is the outermost level.</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 class="std std-ref">output commands</span></a>. The scalar is the potential
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energy discussed above. The vector is the total force added to the
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group of atoms. The scalar and vector values calculated by this fix
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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"><span class="doc">run</span></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"><span class="doc">minimize</span></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"><span class="doc">fix_modify</span></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</h2>
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<p>This fix is part of the MISC package. It is only enabled if LAMMPS
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was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section for more info.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<p><a class="reference internal" href="fix_addforce.html"><span class="doc">fix addforce</span></a></p>
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<p><strong>Default:</strong> none</p>
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