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<title>fix qbmsst command &mdash; LAMMPS documentation</title>
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<li class="toctree-l1"><a class="reference internal" href="Section_commands.html">3. Commands</a></li>
<li class="toctree-l1"><a class="reference internal" href="Section_packages.html">4. Packages</a></li>
<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="tutorials.html">Tutorials</a></li>
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<li class="toctree-l1 current"><a class="reference internal" href="fixes.html">Fixes</a><ul class="current">
<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_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_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_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_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_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_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_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_deform.html">fix deform command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_deform.html#fix-deform-kk-command">fix deform/kk 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_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"><a class="reference internal" href="fix_efield.html">fix efield command</a></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_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_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_gravity.html#fix-gravity-omp-command">fix gravity/omp 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_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.html#fix-langevin-kk-command">fix langevin/kk 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_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_pc.html">fix lb/pc command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_lb_rigid_pc_sphere.html">fix lb/rigid/pc/sphere 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_meso.html">fix meso command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_meso_stationary.html">fix meso/stationary 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_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_neb.html">fix neb 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-nvt-intel-command">fix nvt/intel command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-nvt-kk-command">fix nvt/kk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-nvt-omp-command">fix nvt/omp 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-npt-intel-command">fix npt/intel command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-npt-kk-command">fix npt/kk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-npt-omp-command">fix npt/omp 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.html#fix-nph-kk-command">fix nph/kk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nh.html#fix-nph-omp-command">fix nph/omp 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_nph_asphere.html">fix nph/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nph_asphere.html#fix-nph-asphere-omp-command">fix nph/asphere/omp 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_nph_sphere.html#fix-nph-sphere-omp-command">fix nph/sphere/omp 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_nphug.html#fix-nphug-omp-command">fix nphug/omp 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_asphere.html#fix-npt-asphere-omp-command">fix npt/asphere/omp 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_sphere.html">fix npt/sphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_npt_sphere.html#fix-npt-sphere-omp-command">fix npt/sphere/omp 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.html#fix-nve-intel-command">fix nve/intel command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve.html#fix-nve-kk-command">fix nve/kk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nve.html#fix-nve-omp-command">fix nve/omp 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.html#fix-nve-asphere-intel-command">fix nve/asphere/intel 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_body.html">fix nve/body 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_sphere.html#fix-nve-sphere-omp-command">fix nve/sphere/omp 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_nvt_asphere.html">fix nvt/asphere command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_asphere.html#fix-nvt-asphere-omp-command">fix nvt/asphere/omp 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.html#fix-nvt-sllod-intel-command">fix nvt/sllod/intel command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_nvt_sllod.html#fix-nvt-sllod-omp-command">fix nvt/sllod/omp 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_nvt_sphere.html#fix-nvt-sphere-omp-command">fix nvt/sphere/omp 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_phonon.html">fix phonon command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_pimd.html">fix pimd command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_planeforce.html">fix planeforce command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_poems.html">fix poems</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_pour.html">fix pour command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_press_berendsen.html">fix press/berendsen command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_print.html">fix print command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_property_atom.html">fix property/atom command</a></li>
<li class="toctree-l2 current"><a class="current reference internal" href="#">fix qbmsst 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="#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_qeq.html">fix qeq/point command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq.html#fix-qeq-shielded-command">fix qeq/shielded command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq.html#fix-qeq-slater-command">fix qeq/slater command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq.html#fix-qeq-dynamic-command">fix qeq/dynamic command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq.html#fix-qeq-fire-command">fix qeq/fire command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq_comb.html">fix qeq/comb command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq_comb.html#fix-qeq-comb-omp-command">fix qeq/comb/omp command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq_reax.html">fix qeq/reax command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qeq_reax.html#fix-qeq-reax-kk-command">fix qeq/reax/kk command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qmmm.html">fix qmmm command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_qtb.html">fix qtb command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_reax_bonds.html">fix reax/bonds command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_reax_bonds.html#fix-reax-c-bonds-command">fix reax/c/bonds command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_reaxc_species.html">fix reax/c/species command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_recenter.html">fix recenter command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_restrain.html">fix restrain command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html">fix rigid command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-nve-command">fix rigid/nve command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-nvt-command">fix rigid/nvt command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-npt-command">fix rigid/npt command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-nph-command">fix rigid/nph command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-small-command">fix rigid/small command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-nve-small-command">fix rigid/nve/small command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-nvt-small-command">fix rigid/nvt/small command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-npt-small-command">fix rigid/npt/small command</a></li>
<li class="toctree-l2"><a class="reference internal" href="fix_rigid.html#fix-rigid-nph-small-command">fix rigid/nph/small command</a></li>
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<div class="section" id="fix-qbmsst-command">
<span id="index-0"></span><h1>fix qbmsst command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<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">qbmsst</span> <span class="nb">dir</span> <span class="n">shockvel</span> <span class="n">keyword</span> <span class="n">value</span> <span class="o">...</span>
</pre></div>
</div>
<ul>
<li><p class="first">ID, group-ID are documented in <a class="reference internal" href="fix.html"><span class="doc">fix</span></a> command</p>
</li>
<li><p class="first">qbmsst = style name of this fix</p>
</li>
<li><p class="first">dir = <em>x</em> or <em>y</em> or <em>z</em></p>
</li>
<li><p class="first">shockvel = shock velocity (strictly positive, velocity units)</p>
</li>
<li><p class="first">zero or more keyword/value pairs may be appended</p>
</li>
<li><p class="first">keyword = <em>q</em> or <em>mu</em> or <em>p0</em> or <em>v0</em> or <em>e0</em> or <em>tscale</em> or <em>damp</em> or <em>seed</em>or <em>f_max</em> or <em>N_f</em> or <em>eta</em> or <em>beta</em> or <em>T_init</em></p>
<pre class="literal-block">
<em>q</em> value = cell mass-like parameter (mass^2/distance^4 units)
<em>mu</em> value = artificial viscosity (mass/distance/time units)
<em>p0</em> value = initial pressure in the shock equations (pressure units)
<em>v0</em> value = initial simulation cell volume in the shock equations (distance^3 units)
<em>e0</em> value = initial total energy (energy units)
<em>tscale</em> value = reduction in initial temperature (unitless fraction between 0.0 and 1.0)
<em>damp</em> value = damping parameter (time units) inverse of friction &lt;i&gt;&amp;gamma;&lt;/i&gt;
<em>seed</em> value = random number seed (positive integer)
<em>f_max</em> value = upper cutoff frequency of the vibration spectrum (1/time units)
<em>N_f</em> value = number of frequency bins (positive integer)
<em>eta</em> value = coupling constant between the shock system and the quantum thermal bath (positive unitless)
<em>beta</em> value = the quantum temperature is updated every beta time steps (positive integer)
<em>T_init</em> value = quantum temperature for the initial state (temperature units)
</pre>
</li>
</ul>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<pre class="literal-block">
fix 1 all qbmsst z 0.122 q 25 mu 0.9 tscale 0.01 damp 200 seed 35082 f_max 0.3 N_f 100 eta 1 beta 400 T_init 110 (liquid methane modeled with the REAX force field, real units)
fix 2 all qbmsst z 72 q 40 tscale 0.05 damp 1 seed 47508 f_max 120.0 N_f 100 eta 1.0 beta 500 T_init 300 (quartz modeled with the BKS force field, metal units)
</pre>
<p>Two example input scripts are given, including shocked alpha quartz
and shocked liquid methane. The input script first equilibrate an
initial state with the quantum thermal bath at the target temperature
and then apply the qbmsst to simulate shock compression with quantum
nuclear correction. The following two figures plot related quantities
for shocked alpha quartz.</p>
<img alt="_images/qbmsst_init.jpg" class="align-center" src="_images/qbmsst_init.jpg" />
<p>Figure 1. Classical temperature &lt;i&gt;T&lt;/i&gt;&lt;sup&gt;cl&lt;/sup&gt; = &amp;sum;
&lt;i&gt;m&lt;sub&gt;i&lt;/sub&gt;v&lt;sub&gt;i&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;/3Nk&lt;/i&gt;&lt;sub&gt;B&lt;/sub&gt; vs. time
for coupling the alpha quartz initial state with the quantum thermal
bath at target quantum temperature &lt;i&gt;T&lt;/i&gt;&lt;sup&gt;qm&lt;/sup&gt; = 300 K. The
NpH ensemble is used for time integration while QTB provides the
colored random force. &lt;i&gt;T&lt;/i&gt;&lt;sup&gt;cl&lt;/sup&gt; converges at the timescale
of <em>damp</em> which is set to be 1 ps.</p>
<img alt="_images/qbmsst_shock.jpg" class="align-center" src="_images/qbmsst_shock.jpg" />
<p>Figure 2. Quantum temperature and pressure vs. time for simulating
shocked alpha quartz with the QBMSST. The shock propagates along the z
direction. Restart of the QBMSST command is demonstrated in the
example input script. Thermodynamic quantities stay continuous before
and after the restart.</p>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>This command performs the Quantum-Bath coupled Multi-Scale Shock
Technique (QBMSST) integration. See <a class="reference internal" href="#qi"><span class="std std-ref">(Qi)</span></a> for a detailed
description of this method. The QBMSST provides description of the
thermodynamics and kinetics of shock processes while incorporating
quantum nuclear effects. The <em>shockvel</em> setting determines the steady
shock velocity that will be simulated along direction <em>dir</em>.</p>
<p>Quantum nuclear effects <a class="reference internal" href="fix_qtb.html"><span class="doc">(fix qtb)</span></a> can be crucial
especially when the temperature of the initial state is below the
classical limit or there is a great change in the zero point energies
between the initial and final states. Theoretical post processing
quantum corrections of shock compressed water and methane have been
reported as much as 30% of the temperatures <a class="reference internal" href="#goldman"><span class="std std-ref">(Goldman)</span></a>. A
self-consistent method that couples the shock to a quantum thermal
bath described by a colored noise Langevin thermostat has been
developed by Qi et al <a class="reference internal" href="#qi"><span class="std std-ref">(Qi)</span></a> and applied to shocked methane. The
onset of chemistry is reported to be at a pressure on the shock
Hugoniot that is 40% lower than observed with classical molecular
dynamics.</p>
<p>It is highly recommended that the system be already in an equilibrium
state with a quantum thermal bath at temperature of <em>T_init</em>. The fix
command <a class="reference internal" href="fix_qtb.html"><span class="doc">fix qtb</span></a> at constant temperature <em>T_init</em> could
be used before applying this command to introduce self-consistent
quantum nuclear effects into the initial state.</p>
<p>The parameters <em>q</em>, <em>mu</em>, <em>e0</em>, <em>p0</em>, <em>v0</em> and <em>tscale</em> are described
in the command <a class="reference internal" href="fix_msst.html"><span class="doc">fix msst</span></a>. The values of <em>e0</em>, <em>p0</em>, or
<em>v0</em> will be calculated on the first step if not specified. The
parameter of <em>damp</em>, <em>f_max</em>, and <em>N_f</em> are described in the command
<a class="reference internal" href="fix_qtb.html"><span class="doc">fix qtb</span></a>.</p>
<p>The fix qbmsst command couples the shock system to a quantum thermal
bath with a rate that is proportional to the change of the total
energy of the shock system, &lt;i&gt;etot&lt;/i&gt; - &lt;i&gt;etot&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;.
Here &lt;i&gt;etot&lt;/i&gt; consists of both the system energy and a thermal
term, see <a class="reference internal" href="#qi"><span class="std std-ref">(Qi)</span></a>, and &lt;i&gt;etot&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt; = <em>e0</em> is the
initial total energy.</p>
<p>The <em>eta</em> (&lt;i&gt;&amp;eta;&lt;/i&gt;) parameter is a unitless coupling constant
between the shock system and the quantum thermal bath. A small <em>eta</em>
value cannot adjust the quantum temperature fast enough during the
temperature ramping period of shock compression while large <em>eta</em>
leads to big temperature oscillation. A value of <em>eta</em> between 0.3 and
1 is usually appropriate for simulating most systems under shock
compression. We observe that different values of <em>eta</em> lead to almost
the same final thermodynamic state behind the shock, as expected.</p>
<p>The quantum temperature is updated every <em>beta</em> (&lt;i&gt;&amp;beta;&lt;/i&gt;) steps
with an integration time interval <em>beta</em> times longer than the
simulation time step. In that case, &lt;i&gt;etot&lt;/i&gt; is taken as its
average over the past <em>beta</em> steps. The temperature of the quantum
thermal bath &lt;i&gt;T&lt;/i&gt;&lt;sup&gt;qm&lt;/sup&gt; changes dynamically according to
the following equation where &amp;Delta;&lt;i&gt;t&lt;/i&gt; is the MD time step and
&lt;i&gt;&amp;gamma;&lt;/i&gt; is the friction constant which is equal to the inverse
of the <em>damp</em> parameter.</p>
<center><font size="4"> <i>dT</i><sup>qm</sup>/<i>dt =
&gamma;&eta;</i>&sum;<i><sup>&beta;</sup><sub>l =
1</sub></i>[<i>etot</i>(<i>t-l</i>&Delta;<i>t</i>)-<i>etot</i><sub>0</sub>]/<i>3&beta;Nk</i><sub>B</sub>
</font></center><p>The parameter <em>T_init</em> is the initial temperature of the quantum
thermal bath and the system before shock loading.</p>
<p>For all pressure styles, the simulation box stays orthorhombic in
shape. Parrinello-Rahman boundary conditions (tilted box) are
supported by LAMMPS, but are not implemented for QBMSST.</p>
<hr class="docutils" />
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>Because the state of the random number generator is not written to
<a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>, this fix cannot be restarted
&#8220;exactly&#8221; in an uninterrupted fashion. However, in a statistical
sense, a restarted simulation should produce similar behaviors of the
system as if it is not interrupted. To achieve such a restart, one
should write explicitly the same value for <em>q</em>, <em>mu</em>, <em>damp</em>, <em>f_max</em>,
<em>N_f</em>, <em>eta</em>, and <em>beta</em> and set <em>tscale</em> = 0 if the system is
compressed during the first run.</p>
<p>The progress of the QBMSST can be monitored by printing the global
scalar and global vector quantities computed by the fix. The global
vector contains five values in this order:</p>
<p>[<em>dhugoniot</em>, <em>drayleigh</em>, <em>lagrangian_speed</em>, <em>lagrangian_position</em>,
<em>quantum_temperature</em>]</p>
<ol class="arabic simple">
<li><em>dhugoniot</em> is the departure from the Hugoniot (temperature units).</li>
<li><em>drayleigh</em> is the departure from the Rayleigh line (pressure units).</li>
<li><em>lagrangian_speed</em> is the laboratory-frame Lagrangian speed (particle velocity) of the computational cell (velocity units).</li>
<li><em>lagrangian_position</em> is the computational cell position in the reference frame moving at the shock speed. This is the distance of the computational cell behind the shock front.</li>
<li><em>quantum_temperature</em> is the temperature of the quantum thermal bath &lt;i&gt;T&lt;/i&gt;&lt;sup&gt;qm&lt;/sup&gt;.</li>
</ol>
<p>To print these quantities to the log file with descriptive column
headers, the following LAMMPS commands are suggested. Here the
<a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> energy command is also enabled to allow
the thermo keyword <em>etotal</em> to print the quantity &lt;i&gt;etot&lt;/i&gt;. See
also the <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> command.</p>
<pre class="literal-block">
fix fix_id all msst z
fix_modify fix_id energy yes
variable dhug equal f_fix_id[1]
variable dray equal f_fix_id[2]
variable lgr_vel equal f_fix_id[3]
variable lgr_pos equal f_fix_id[4]
variable T_qm equal f_fix_id[5]
thermo_style custom step temp ke pe lz pzz etotal v_dhug v_dray v_lgr_vel v_lgr_pos v_T_qm f_fix_id
</pre>
<p>The global scalar under the entry f_fix_id is the quantity of thermo
energy as an extra part of &lt;i&gt;etot&lt;/i&gt;. This global scalar and the
vector of 5 quantities can be accessed by various <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. It is worth noting that the
temp keyword under the <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> command print
the instantaneous classical temperature &lt;i&gt;T&lt;/i&gt;&lt;sup&gt;cl&lt;/sup&gt; as
described in the command <a class="reference internal" href="fix_qtb.html"><span class="doc">fix qtb</span></a>.</p>
</div>
<hr class="docutils" />
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<p>This fix style is part of the USER-QTB package. It is only enabled if
LAMMPS 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>
<p>All cell dimensions must be periodic. This fix can not be used with a
triclinic cell. The QBMSST fix has been tested only for the group-ID
all.</p>
</div>
<hr class="docutils" />
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="fix_qtb.html"><span class="doc">fix qtb</span></a>, <a class="reference internal" href="fix_msst.html"><span class="doc">fix msst</span></a></p>
</div>
<hr class="docutils" />
<div class="section" id="default">
<h2>Default</h2>
<p>The keyword defaults are q = 10, mu = 0, tscale = 0.01, damp = 1, seed
= 880302, f_max = 200.0, N_f = 100, eta = 1.0, beta = 100, and
T_init=300.0. e0, p0, and v0 are calculated on the first step.</p>
<hr class="docutils" />
<p id="goldman"><strong>(Goldman)</strong> Goldman, Reed and Fried, J. Chem. Phys. 131, 204103 (2009)</p>
<p id="qi"><strong>(Qi)</strong> Qi and Reed, J. Phys. Chem. A 116, 10451 (2012).</p>
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