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lammps/doc/html/fix_efield.html
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<title>fix efield command &mdash; LAMMPS documentation</title>
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<div class="lammps_version">Version: <b>19 Nov 2024</b></div>
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<li class="toctree-l1"><a class="reference internal" href="Intro.html">1. Introduction</a></li>
<li class="toctree-l1"><a class="reference internal" href="Install.html">2. Install LAMMPS</a></li>
<li class="toctree-l1"><a class="reference internal" href="Build.html">3. Build LAMMPS</a></li>
<li class="toctree-l1"><a class="reference internal" href="Run_head.html">4. Run LAMMPS</a></li>
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<p class="caption" role="heading"><span class="caption-text">Programmer Guide</span></p>
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<li class="toctree-l1"><a class="reference internal" href="Library.html">1. LAMMPS Library Interfaces</a></li>
<li class="toctree-l1"><a class="reference internal" href="Python_head.html">2. Use Python with LAMMPS</a></li>
<li class="toctree-l1"><a class="reference internal" href="Modify.html">3. Modifying &amp; extending LAMMPS</a></li>
<li class="toctree-l1"><a class="reference internal" href="Developer.html">4. Information for Developers</a></li>
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<p class="caption" role="heading"><span class="caption-text">Command Reference</span></p>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="commands_list.html">Commands</a></li>
<li class="toctree-l1 current"><a class="reference internal" href="fixes.html">Fix Styles</a><ul class="current">
<li class="toctree-l2"><a class="reference internal" href="fix_accelerate_cos.html">fix accelerate/cos command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_acks2_reaxff.html">fix acks2/reaxff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_adapt.html">fix adapt command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_adapt_fep.html">fix adapt/fep command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_add_heat.html">fix add/heat command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_addforce.html">fix addforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_addtorque.html">fix addtorque command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_alchemy.html">fix alchemy command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_amoeba_bitorsion.html">fix amoeba/bitorsion command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_amoeba_pitorsion.html">fix amoeba/pitorsion command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_append_atoms.html">fix append/atoms command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_atc.html">fix atc command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_atom_swap.html">fix atom/swap command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_atom.html">fix ave/atom command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_chunk.html">fix ave/chunk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_correlate.html">fix ave/correlate command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_correlate_long.html">fix ave/correlate/long command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_grid.html">fix ave/grid command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_histo.html">fix ave/histo command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_histo.html#fix-ave-histo-weight-command">fix ave/histo/weight command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_spatial.html">fix ave/spatial command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_spatial_sphere.html">fix ave/spatial/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ave_time.html">fix ave/time command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_aveforce.html">fix aveforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_balance.html">fix balance command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_bocs.html">fix bocs command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_bond_break.html">fix bond/break command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_bond_create.html">fix bond/create command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_bond_create.html#fix-bond-create-angle-command">fix bond/create/angle command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_bond_react.html">fix bond/react command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_bond_swap.html">fix bond/swap command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_box_relax.html">fix box/relax command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_brownian.html">fix brownian command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_brownian.html#fix-brownian-sphere-command">fix brownian/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_brownian.html#fix-brownian-asphere-command">fix brownian/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_charge_regulation.html">fix charge/regulation command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_cmap.html">fix cmap command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_colvars.html">fix colvars command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_controller.html">fix controller command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_damping_cundall.html">fix damping/cundall command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_deform.html">fix deform command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_deform_pressure.html">fix deform/pressure command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_deposit.html">fix deposit command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_dpd_energy.html">fix dpd/energy command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_dpd_source.html">fix edpd/source command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_dpd_source.html#fix-tdpd-source-command">fix tdpd/source command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_drag.html">fix drag command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_drude.html">fix drude command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_drude_transform.html">fix drude/transform/direct command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_drude_transform.html#fix-drude-transform-inverse-command">fix drude/transform/inverse command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_dt_reset.html">fix dt/reset command</a></li>
<li class="toctree-l2 current"><a class="current reference internal" href="#">fix efield command</a></li>
<li class="toctree-l2"><a class="reference internal" href="#fix-efield-tip4p-command">fix efield/tip4p command</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#syntax">Syntax</a></li>
<li class="toctree-l3"><a class="reference internal" href="#examples">Examples</a></li>
<li class="toctree-l3"><a class="reference internal" href="#description">Description</a></li>
<li class="toctree-l3"><a class="reference internal" href="#restart-fix-modify-output-run-start-stop-minimize-info">Restart, fix_modify, output, run start/stop, minimize info</a></li>
<li class="toctree-l3"><a class="reference internal" href="#restrictions">Restrictions</a></li>
<li class="toctree-l3"><a class="reference internal" href="#related-commands">Related commands</a></li>
<li class="toctree-l3"><a class="reference internal" href="#default">Default</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="fix_ehex.html">fix ehex command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_electrode.html">fix electrode/conp command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_electrode.html#fix-electrode-conq-command">fix electrode/conq command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_electrode.html#fix-electrode-thermo-command">fix electrode/thermo command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_electron_stopping.html">fix electron/stopping command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_electron_stopping.html#fix-electron-stopping-fit-command">fix electron/stopping/fit command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_enforce2d.html">fix enforce2d command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_eos_cv.html">fix eos/cv command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_eos_table.html">fix eos/table command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_eos_table_rx.html">fix eos/table/rx command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_evaporate.html">fix evaporate command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_external.html">fix external command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ffl.html">fix ffl command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_filter_corotate.html">fix filter/corotate command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_flow_gauss.html">fix flow/gauss command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_freeze.html">fix freeze command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_gcmc.html">fix gcmc command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_gld.html">fix gld command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_gle.html">fix gle command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_gravity.html">fix gravity command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_grem.html">fix grem command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_halt.html">fix halt command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_heat.html">fix heat command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_heat_flow.html">fix heat/flow command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_hyper_global.html">fix hyper/global command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_hyper_local.html">fix hyper/local command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_imd.html">fix imd command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_indent.html">fix indent command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_ipi.html">fix ipi command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_langevin.html">fix langevin command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_langevin_drude.html">fix langevin/drude command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_langevin_eff.html">fix langevin/eff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_langevin_spin.html">fix langevin/spin command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_lb_fluid.html">fix lb/fluid command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_lb_momentum.html">fix lb/momentum command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_lb_viscous.html">fix lb/viscous command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_lineforce.html">fix lineforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_manifoldforce.html">fix manifoldforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_mdi_qm.html">fix mdi/qm command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_mdi_qmmm.html">fix mdi/qmmm command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_meso_move.html">fix meso/move command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_mol_swap.html">fix mol/swap command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_momentum.html">fix momentum command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_momentum.html#fix-momentum-chunk-command">fix momentum/chunk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_move.html">fix move command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_msst.html">fix msst command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_mvv_dpd.html">fix mvv/dpd command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_mvv_dpd.html#fix-mvv-edpd-command">fix mvv/edpd command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_mvv_dpd.html#fix-mvv-tdpd-command">fix mvv/tdpd command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_neb.html">fix neb command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_neb_spin.html">fix neb/spin command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html">fix nvt command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-npt-command">fix npt command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-nph-command">fix nph command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh_eff.html">fix nvt/eff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh_eff.html#fix-npt-eff-command">fix npt/eff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh_eff.html#fix-nph-eff-command">fix nph/eff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh_uef.html">fix nvt/uef command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh_uef.html#fix-npt-uef-command">fix npt/uef command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nonaffine_displacement.html">fix nonaffine/displacement command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nph_asphere.html">fix nph/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nph_body.html">fix nph/body command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nph_sphere.html">fix nph/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nphug.html">fix nphug command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_npt_asphere.html">fix npt/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_npt_body.html">fix npt/body command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_npt_cauchy.html">fix npt/cauchy command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_npt_sphere.html">fix npt/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_numdiff.html">fix numdiff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_numdiff_virial.html">fix numdiff/virial command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve.html">fix nve command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_asphere.html">fix nve/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_asphere_noforce.html">fix nve/asphere/noforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_awpmd.html">fix nve/awpmd command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_body.html">fix nve/body command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_bpm_sphere.html">fix nve/bpm/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_dot.html">fix nve/dot command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_dotc_langevin.html">fix nve/dotc/langevin command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_eff.html">fix nve/eff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_limit.html">fix nve/limit command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_line.html">fix nve/line command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_manifold_rattle.html">fix nve/manifold/rattle command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_noforce.html">fix nve/noforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_sphere.html">fix nve/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_spin.html">fix nve/spin command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve_tri.html">fix nve/tri command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvk.html">fix nvk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_asphere.html">fix nvt/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_body.html">fix nvt/body command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_manifold_rattle.html">fix nvt/manifold/rattle command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_sllod.html">fix nvt/sllod command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_sllod_eff.html">fix nvt/sllod/eff command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_sphere.html">fix nvt/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_oneway.html">fix oneway command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_orient.html">fix orient/fcc command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_orient.html#fix-orient-bcc-command">fix orient/bcc command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_orient_eco.html">fix orient/eco command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="fix_phonon.html">fix phonon command</a></li>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
<section id="fix-efield-command">
<span id="index-2"></span><span id="index-1"></span><span id="index-0"></span><h1>fix efield command<a class="headerlink" href="#fix-efield-command" title="Link to this heading"></a></h1>
</section>
<section id="fix-efield-tip4p-command">
<h1>fix efield/tip4p command<a class="headerlink" href="#fix-efield-tip4p-command" title="Link to this heading"></a></h1>
<section id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Link to this heading"></a></h2>
<div class="highlight-LAMMPS notranslate"><div class="highlight"><pre><span></span><span class="k">fix </span><span class="nv nv-Identifier">ID</span><span class="w"> </span><span class="nv nv-Identifier">group-ID</span><span class="w"> </span><span class="n">style</span><span class="w"> </span><span class="n">ex</span><span class="w"> </span><span class="n">ey</span><span class="w"> </span><span class="n">ez</span><span class="w"> </span><span class="n">keyword</span><span class="w"> </span><span class="n">value</span><span class="w"> </span><span class="n">...</span>
</pre></div>
</div>
<ul>
<li><p>ID, group-ID are documented in <a class="reference internal" href="fix.html"><span class="doc">fix</span></a> command</p></li>
<li><p>style = <em>efield</em> or <em>efield/tip4p</em></p></li>
<li><p>ex,ey,ez = E-field component values (electric field units)</p></li>
<li><p>any of ex,ey,ez can be a variable (see below)</p></li>
<li><p>zero or more keyword/value pairs may be appended to args</p></li>
<li><p>keyword = <em>region</em> or <em>energy</em> or <em>potential</em></p>
<pre class="literal-block"><em>region</em> value = region-ID
region-ID = ID of region atoms must be in to have added force
<em>energy</em> value = v_name
v_name = variable with name that calculates the potential energy of each atom in the added E-field
<em>potential</em> value = v_name
v_name = variable with name that calculates the electric potential of each atom in the added E-field</pre>
</li>
</ul>
</section>
<section id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Link to this heading"></a></h2>
<div class="highlight-LAMMPS notranslate"><div class="highlight"><pre><span></span><span class="k">fix </span><span class="nv nv-Identifier">kick</span><span class="w"> </span><span class="nv nv-Identifier">external-field</span><span class="w"> </span><span class="n">efield</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">0.0</span><span class="w"> </span><span class="m">0.0</span>
<span class="k">fix </span><span class="nv nv-Identifier">kick</span><span class="w"> </span><span class="nv nv-Identifier">external-field</span><span class="w"> </span><span class="n">efield</span><span class="w"> </span><span class="m">0.0</span><span class="w"> </span><span class="m">0.0</span><span class="w"> </span><span class="n">v_oscillate</span>
<span class="k">fix </span><span class="nv nv-Identifier">kick</span><span class="w"> </span><span class="nv nv-Identifier">external-field</span><span class="w"> </span><span class="n">efield</span><span class="o">/</span><span class="n">tip4p</span><span class="w"> </span><span class="m">1.0</span><span class="w"> </span><span class="m">0.0</span><span class="w"> </span><span class="m">0.0</span>
</pre></div>
</div>
</section>
<section id="description">
<h2>Description<a class="headerlink" href="#description" title="Link to this heading"></a></h2>
<p>Add a force <span class="math notranslate nohighlight">\(\vec{F} = q\vec{E}\)</span> to each charged atom in the group due to an
external electric field being applied to the system. If the system
contains point-dipoles, also add a torque on the dipoles due to the
external electric field.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 28Mar2023.</span></p>
</div>
<p>When the <em>efield/tip4p</em> style is used, the E-field will be applied to
the position of the virtual charge site M of a TIP4P molecule instead of
the oxygen position as it is defined by a corresponding <a class="reference internal" href="pair_lj_cut_tip4p.html"><span class="doc">TIP4P pair
style</span></a>. The forces on the M site due to the
external field are projected on the oxygen and hydrogen atoms of the
TIP4P molecules.</p>
<p>For charges, any of the 3 quantities defining the E-field components can
be specified as an equal-style or atom-style <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 variable, it should be
specified as v_name, where name is the variable name. In this case, the
variable will be evaluated each timestep, and its value used to
determine the E-field component.</p>
<p>For point-dipoles, equal-style variables can be used, but atom-style
variables are not currently supported, since they imply a spatial
gradient in the electric field which means additional terms with
gradients of the field are required for the force and torque on dipoles.</p>
<p>Equal-style variables can specify formulas with various mathematical
functions, and include <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> command
keywords for the simulation box parameters and timestep and elapsed
time. Thus it is easy to specify a time-dependent E-field.</p>
<p>Atom-style variables can specify the same formulas as equal-style
variables but can also include per-atom values, such as atom
coordinates. Thus it is easy to specify a spatially-dependent E-field
with optional time-dependence as well.</p>
<p>If the <em>region</em> keyword is used, the atom must also be in the
specified geometric <a class="reference internal" href="region.html"><span class="doc">region</span></a> in order to have force added
to it.</p>
<hr class="docutils" />
<p>Adding a force or torque to atoms implies a change in their potential
energy as they move or rotate due to the applied E-field.</p>
<p>For dynamics via the “run” command, this energy can be optionally
added to the systems potential energy for thermodynamic output (see
below). For energy minimization via the “minimize” command, this
energy must be added to the systems potential energy to formulate a
self-consistent minimization problem (see below).</p>
<p>The <em>energy</em> keyword is not allowed if the added field is a constant
vector (ex,ey,ez), with all components defined as numeric constants
and not as variables. This is because LAMMPS can compute the energy
for each charged particle directly as</p>
<div class="math notranslate nohighlight">
\[U_{efield} = -\vec{x} \cdot q\vec{E} = -q (x\cdot E_x + y\cdot E_y + z\cdot Ez),\]</div>
<p>so that <span class="math notranslate nohighlight">\(-\nabla U_{efield} = \vec{F}\)</span>. Similarly for point-dipole particles
the energy can be computed as</p>
<div class="math notranslate nohighlight">
\[U_{efield} = -\vec{\mu} \cdot \vec{E} = -\mu_x\cdot E_x + \mu_y\cdot E_y + \mu_z\cdot E_z\]</div>
<p>The <em>energy</em> keyword is optional if the added force is defined with
one or more variables, and if you are performing dynamics via the
<a class="reference internal" href="run.html"><span class="doc">run</span></a> command. If the keyword is not used, LAMMPS will set
the energy to 0.0, which is typically fine for dynamics.</p>
<p>The <em>energy</em> keyword (or <em>potential</em> keyword, described below)
is required if the added force is defined with
one or more variables, and you are performing energy minimization via
the “minimize” command for charged particles. It is not required for
point-dipoles, but a warning is issued since the minimizer in LAMMPS
does not rotate dipoles, so you should not expect to be able to
minimize the orientation of dipoles in an applied electric field.</p>
<p>The <em>energy</em> keyword specifies the name of an atom-style
<a class="reference internal" href="variable.html"><span class="doc">variable</span></a> which is used to compute the energy of each
atom as function of its position. Like variables used for <em>ex</em>,
<em>ey</em>, <em>ez</em>, the energy variable is specified as “v_name”, where “name”
is the variable name.</p>
<p>Note that when the <em>energy</em> keyword is used during an energy
minimization, you must ensure that the formula defined for the
atom-style <a class="reference internal" href="variable.html"><span class="doc">variable</span></a> is consistent with the force
variable formulas, i.e. that -Grad(E) = F. For example, if the force
due to the electric field were a spring-like F = kx, then the energy
formula should be E = -0.5kx^2. If you dont do this correctly, the
minimization will not converge properly.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 15Jun2023.</span></p>
</div>
<p>The <em>potential</em> keyword can be used as an alternative to the <em>energy</em> keyword
to specify the name of an atom-style variable, which is used to compute the
added electric potential to each atom as a function of its position. The
variable should have units of electric field multiplied by distance (that is,
in <cite>units real</cite>, the potential should be in volts). As with the <em>energy</em>
keyword, the variable name is specified as “v_name”. The energy added by this
fix is then calculated as the electric potential multiplied by charge.</p>
<p>The <em>potential</em> keyword is mainly intended for correct charge
equilibration in simulations with <a class="reference internal" href="fix_qeq_reaxff.html"><span class="doc">fix qeq/reaxff</span></a>,
since with variable charges the electric potential can be known
beforehand but the energy cannot. A small additional benefit is that
the <em>energy</em> keyword requires an additional conversion to energy units
which the <em>potential</em> keyword avoids. Thus, when the <em>potential</em>
keyword is specified, the <em>energy</em> keyword must not be used. As with
<em>energy</em>, the <em>potential</em> keyword is not allowed if the added field is a
constant vector. The <em>potential</em> keyword is not supported by <em>fix
efield/tip4p</em>.</p>
</section>
<hr class="docutils" />
<section id="restart-fix-modify-output-run-start-stop-minimize-info">
<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Link to this heading"></a></h2>
<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>
<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
fix to add the potential energy inferred by the added force due to the
electric field to the global potential energy of the system as part of
<a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>. The default setting for
this fix is <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify energy no</span></a>. Note that this
energy is a fictitious quantity but is needed so that the <a class="reference internal" href="minimize.html"><span class="doc">minimize</span></a> command can include the forces added by this fix in a
consistent manner. I.e. there is a decrease in potential energy when
atoms move in the direction of the added force due to the electric
field.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>virial</em> option is supported by this
fix to add the contribution due to the added forces on atoms to both the
global pressure and per-atom stress of the system via the <a class="reference internal" href="compute_pressure.html"><span class="doc">compute
pressure</span></a> and <a class="reference internal" href="compute_stress_atom.html"><span class="doc">compute stress/atom</span></a> commands. The former can be accessed by
<a class="reference internal" href="thermo_style.html"><span class="doc">thermodynamic output</span></a>. The default setting for
this fix is <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify virial no</span></a>.</p>
<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
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>
integrator the fix adding its forces. Default is the outermost level.</p>
<p>This fix computes a global scalar and a global 3-vector of forces,
which can be accessed by various <a class="reference internal" href="Howto_output.html"><span class="doc">output commands</span></a>. The scalar is the potential energy discussed above.
The vector is the total force added to the group of atoms. The scalar
and vector values calculated by this fix are “extensive”.</p>
<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
the <a class="reference internal" href="run.html"><span class="doc">run</span></a> command.</p>
<p>The forces due to this fix are imposed during an energy minimization,
invoked by the <a class="reference internal" href="minimize.html"><span class="doc">minimize</span></a> command. You should not
specify force components with a variable that has time-dependence for
use with a minimizer, since the minimizer increments the timestep as
the iteration count during the minimization.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>If you want the fictitious potential energy associated with the
added forces to be included in the total potential energy of the
system (the quantity being minimized), you MUST enable the
<a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>energy</em> option for this fix.</p>
</div>
<hr class="docutils" />
<p>Styles with a <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed on the <a class="reference internal" href="Speed_packages.html"><span class="doc">Accelerator packages</span></a>
page. The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.</p>
<p>These accelerated styles are part of the GPU, INTEL, KOKKOS,
OPENMP, and OPT packages, respectively. They are only enabled if
LAMMPS was built with those packages. See the <a class="reference internal" href="Build_package.html"><span class="doc">Build package</span></a> page for more info.</p>
<p>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <a class="reference internal" href="Run_options.html"><span class="doc">-suffix command-line switch</span></a> when you invoke LAMMPS, or you can use the
<a class="reference internal" href="suffix.html"><span class="doc">suffix</span></a> command in your input script.</p>
<p>See the <a class="reference internal" href="Speed_packages.html"><span class="doc">Accelerator packages</span></a> page for more
instructions on how to use the accelerated styles effectively.</p>
</section>
<hr class="docutils" />
<section id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Link to this heading"></a></h2>
<p>Fix style <em>efield/tip4p</em> is part of the EXTRA-FIX package. It is only
enabled if LAMMPS was built with that package. See the <a class="reference internal" href="Build_package.html"><span class="doc">Build
package</span></a> page for more info.</p>
<p>Fix style <em>efield/tip4p</em> can only be used with tip4p pair styles.</p>
</section>
<section id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Link to this heading"></a></h2>
<p><a class="reference internal" href="fix_addforce.html"><span class="doc">fix addforce</span></a></p>
</section>
<section id="default">
<h2>Default<a class="headerlink" href="#default" title="Link to this heading"></a></h2>
<p>none</p>
</section>
</section>
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