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<title>3.9. Fix styles — LAMMPS documentation</title>
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<div class="lammps_version">Version: <b>19 Nov 2024</b></div>
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<p class="caption" role="heading"><span class="caption-text">User Guide</span></p>
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<li class="toctree-l1"><a class="reference internal" href="Intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Install.html">2. Install LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Build.html">3. Build LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Run_head.html">4. Run LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Commands.html">5. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Packages.html">6. Optional packages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Speed.html">7. Accelerate performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Howto.html">8. Howto discussions</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Examples.html">9. Example scripts</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Tools.html">10. Auxiliary tools</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Errors.html">11. Errors</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>
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<li class="toctree-l1"><a class="reference internal" href="Python_head.html">2. Use Python with LAMMPS</a></li>
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<li class="toctree-l1 current"><a class="reference internal" href="Modify.html">3. Modifying & extending LAMMPS</a><ul class="current">
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<li class="toctree-l2"><a class="reference internal" href="Modify_overview.html">3.1. Overview</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_contribute.html">3.2. Submitting new features for inclusion in LAMMPS</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_requirements.html">3.3. Requirements for contributions to LAMMPS</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_style.html">3.4. LAMMPS programming style</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_atom.html">3.5. Atom styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_pair.html">3.6. Pair styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_bond.html">3.7. Bond, angle, dihedral, improper styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_compute.html">3.8. Compute styles</a></li>
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<li class="toctree-l2 current"><a class="current reference internal" href="#">3.9. Fix styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_command.html">3.10. Input script command style</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_dump.html">3.11. Dump styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_kspace.html">3.12. Kspace styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_min.html">3.13. Minimization styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_region.html">3.14. Region styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_body.html">3.15. Body styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_gran_sub_mod.html">3.16. Granular Sub-Model styles</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_thermo.html">3.17. Thermodynamic output options</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Modify_variable.html">3.18. Variable options</a></li>
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<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>
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<li class="toctree-l1"><a class="reference internal" href="commands_list.html">Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="fixes.html">Fix Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="computes.html">Compute Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="pairs.html">Pair Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="bonds.html">Bond Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="angles.html">Angle Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dihedrals.html">Dihedral Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="impropers.html">Improper Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dumps.html">Dump Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="fix_modify_atc_commands.html">fix_modify AtC commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Bibliography.html">Bibliography</a></li>
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<a href="Modify_compute.html" class="btn btn-neutral float-left" title="3.8. Compute styles" accesskey="p"><span class="fa fa-arrow-circle-left" aria-hidden="true"></span> Previous</a>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="fix-styles">
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<h1><span class="section-number">3.9. </span>Fix styles<a class="headerlink" href="#fix-styles" title="Link to this heading"></a></h1>
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<p>In LAMMPS, a “fix” is any operation that is computed during
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timestepping that alters some property of the system. Essentially
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everything that happens during a simulation besides force computation,
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neighbor list construction, and output, is a “fix”. This includes
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time integration (update of coordinates and velocities), force
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constraints or boundary conditions (SHAKE or walls), and diagnostics
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(compute a diffusion coefficient). New styles can be created to add
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new options to LAMMPS.</p>
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<p>Fix_setforce.cpp is a simple example of setting forces on atoms to
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prescribed values. There are dozens of fix options already in LAMMPS;
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choose one as a template that is similar to what you want to
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implement.</p>
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<p>Here is a brief description of methods you can define in your new
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derived class. See fix.h for details.</p>
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<table class="docutils align-default">
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<tbody>
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<tr class="row-odd"><td><p>setmask</p></td>
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<td><p>determines when the fix is called during the timestep (required)</p></td>
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</tr>
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<tr class="row-even"><td><p>init</p></td>
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<td><p>initialization before a run (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>init_list</p></td>
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<td><p>store pointer to neighbor list; called by neighbor list code (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>setup_pre_exchange</p></td>
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<td><p>called before atom exchange in setup (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>setup_pre_force</p></td>
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<td><p>called before force computation in setup (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>setup</p></td>
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<td><p>called immediately before the first timestep and after forces are computed (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>min_setup_pre_force</p></td>
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<td><p>like setup_pre_force, but for minimizations instead of MD runs (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>min_setup</p></td>
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<td><p>like setup, but for minimizations instead of MD runs (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>initial_integrate</p></td>
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<td><p>called at very beginning of each timestep (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>pre_exchange</p></td>
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<td><p>called before atom exchange on re-neighboring steps (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>pre_neighbor</p></td>
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<td><p>called before neighbor list build (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>pre_force</p></td>
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<td><p>called before pair & molecular forces are computed (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>post_force</p></td>
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<td><p>called after pair & molecular forces are computed and communicated (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>final_integrate</p></td>
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<td><p>called at end of each timestep (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>end_of_step</p></td>
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<td><p>called at very end of timestep (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>write_restart</p></td>
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<td><p>dumps fix info to restart file (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>restart</p></td>
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<td><p>uses info from restart file to re-initialize the fix (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>grow_arrays</p></td>
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<td><p>allocate memory for atom-based arrays used by fix (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>copy_arrays</p></td>
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<td><p>copy atom info when an atom migrates to a new processor (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>pack_exchange</p></td>
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<td><p>store atom’s data in a buffer (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>unpack_exchange</p></td>
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<td><p>retrieve atom’s data from a buffer (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>pack_restart</p></td>
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<td><p>store atom’s data for writing to restart file (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>unpack_restart</p></td>
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<td><p>retrieve atom’s data from a restart file buffer (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>size_restart</p></td>
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<td><p>size of atom’s data (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>maxsize_restart</p></td>
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<td><p>max size of atom’s data (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>setup_pre_force_respa</p></td>
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<td><p>same as setup_pre_force, but for rRESPA (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>initial_integrate_respa</p></td>
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<td><p>same as initial_integrate, but for rRESPA (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>post_integrate_respa</p></td>
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<td><p>called after the first half integration step is done in rRESPA (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>pre_force_respa</p></td>
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<td><p>same as pre_force, but for rRESPA (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>post_force_respa</p></td>
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<td><p>same as post_force, but for rRESPA (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>final_integrate_respa</p></td>
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<td><p>same as final_integrate, but for rRESPA (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>min_pre_force</p></td>
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<td><p>called after pair & molecular forces are computed in minimizer (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>min_post_force</p></td>
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<td><p>called after pair & molecular forces are computed and communicated in minimizer (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>min_store</p></td>
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<td><p>store extra data for linesearch based minimization on a LIFO stack (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>min_pushstore</p></td>
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<td><p>push the minimization LIFO stack one element down (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>min_popstore</p></td>
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<td><p>pop the minimization LIFO stack one element up (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>min_clearstore</p></td>
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<td><p>clear minimization LIFO stack (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>min_step</p></td>
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<td><p>reset or move forward on line search minimization (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>min_dof</p></td>
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<td><p>report number of degrees of freedom <em>added</em> by this fix in minimization (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>max_alpha</p></td>
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<td><p>report maximum allowed step size during linesearch minimization (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>pack_comm</p></td>
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<td><p>pack a buffer to communicate a per-atom quantity (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>unpack_comm</p></td>
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<td><p>unpack a buffer to communicate a per-atom quantity (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>pack_reverse_comm</p></td>
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<td><p>pack a buffer to reverse communicate a per-atom quantity (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>unpack_reverse_comm</p></td>
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<td><p>unpack a buffer to reverse communicate a per-atom quantity (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>dof</p></td>
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<td><p>report number of degrees of freedom <em>removed</em> by this fix during MD (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>compute_scalar</p></td>
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<td><p>return a global scalar property that the fix computes (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>compute_vector</p></td>
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<td><p>return a component of a vector property that the fix computes (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>compute_array</p></td>
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<td><p>return a component of an array property that the fix computes (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>deform</p></td>
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<td><p>called when the box size is changed (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>reset_target</p></td>
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<td><p>called when a change of the target temperature is requested during a run (optional)</p></td>
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</tr>
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<tr class="row-odd"><td><p>reset_dt</p></td>
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<td><p>is called when a change of the time step is requested during a run (optional)</p></td>
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</tr>
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<tr class="row-even"><td><p>modify_param</p></td>
|
||
<td><p>called when a fix_modify request is executed (optional)</p></td>
|
||
</tr>
|
||
<tr class="row-odd"><td><p>memory_usage</p></td>
|
||
<td><p>report memory used by fix (optional)</p></td>
|
||
</tr>
|
||
<tr class="row-even"><td><p>thermo</p></td>
|
||
<td><p>compute quantities for thermodynamic output (optional)</p></td>
|
||
</tr>
|
||
</tbody>
|
||
</table>
|
||
<p>Typically, only a small fraction of these methods are defined for a
|
||
particular fix. Setmask is mandatory, as it determines when the fix
|
||
will be invoked during the timestep. Fixes that perform time
|
||
integration (<em>nve</em>, <em>nvt</em>, <em>npt</em>) implement initial_integrate() and
|
||
final_integrate() to perform velocity Verlet updates. Fixes that
|
||
constrain forces implement post_force().</p>
|
||
<p>Fixes that perform diagnostics typically implement end_of_step(). For
|
||
an end_of_step fix, one of your fix arguments must be the variable
|
||
“nevery” which is used to determine when to call the fix and you must
|
||
set this variable in the constructor of your fix. By convention, this
|
||
is the first argument the fix defines (after the ID, group-ID, style).</p>
|
||
<p>If the fix needs to store information for each atom that persists from
|
||
timestep to timestep, it can manage that memory and migrate the info
|
||
with the atoms as they move from processors to processor by
|
||
implementing the grow_arrays, copy_arrays, pack_exchange, and
|
||
unpack_exchange methods. Similarly, the pack_restart and
|
||
unpack_restart methods can be implemented to store information about
|
||
the fix in restart files. If you wish an integrator or force
|
||
constraint fix to work with rRESPA (see the <a class="reference internal" href="run_style.html"><span class="doc">run_style</span></a>
|
||
command), the initial_integrate, post_force_integrate, and
|
||
final_integrate_respa methods can be implemented. The thermo method
|
||
enables a fix to contribute values to thermodynamic output, as printed
|
||
quantities and/or to be summed to the potential energy of the system.</p>
|
||
</section>
|
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