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<div class="section" id="fix-tfmc-command">
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<span id="index-0"></span><h1>fix tfmc command<a class="headerlink" href="#fix-tfmc-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID tfmc Delta Temp seed keyword value
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>tfmc = style name of this fix command</li>
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<li>Delta = maximal displacement length (distance units)</li>
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<li>Temp = imposed temperature of the system</li>
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<li>seed = random number seed (positive integer)</li>
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<li>zero or more keyword/arg pairs may be appended</li>
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<li>keyword = <em>com</em> or <em>rot</em></li>
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</ul>
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<pre class="literal-block">
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<em>com</em> args = xflag yflag zflag
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xflag,yflag,zflag = 0/1 to exclude/include each dimension
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<em>rot</em> args = none
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix 1 all tfmc 0.1 1000.0 159345
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fix 1 all tfmc 0.05 600.0 658943 com 1 1 0
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fix 1 all tfmc 0.1 750.0 387068 com 1 1 1 rot
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Perform uniform-acceptance force-bias Monte Carlo (fbMC) simulations,
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using the time-stamped force-bias Monte Carlo (tfMC) algorithm
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described in <a class="reference internal" href="#mees"><span>(Mees)</span></a> and <a class="reference internal" href="#bal"><span>(Bal)</span></a>.</p>
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<p>One successful use case of force-bias Monte Carlo methods is that they
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can be used to extend the time scale of atomistic simulations, in
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particular when long time scale relaxation effects must be considered;
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some interesting examples are given in the review by <a class="reference internal" href="#neyts"><span>(Neyts)</span></a>.
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An example of a typical use case would be the modelling of chemical
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vapour deposition (CVD) processes on a surface, in which impacts by
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gas-phase species can be performed using MD, but subsequent relaxation
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of the surface is too slow to be done using MD only. Using tfMC can
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allow for a much faster relaxation of the surface, so that higher
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fluxes can be used, effectively extending the time scale of the
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simulation. (Such an alternating simulation approach could be set up
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using a <a class="reference internal" href="jump.html"><em>loop</em></a>.)</p>
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<p>The initial version of tfMC algorithm in <a class="reference internal" href="#mees"><span>(Mees)</span></a> contained an
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estimation of the effective time scale of such a simulation, but it
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was later shown that the speed-up one can gain from a tfMC simulation
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is system- and process-dependent, ranging from none to several orders
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of magnitude. In general, solid-state processes such as
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(re)crystallisation or growth can be accelerated by up to two or three
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orders of magnitude, whereas diffusion in the liquid phase is not
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accelerated at all. The observed pseudodynamics when using the tfMC
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method is not the actual dynamics one would obtain using MD, but the
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relative importance of processes can match the actual relative
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dynamics of the system quite well, provided <em>Delta</em> is chosen with
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care. Thus, the system’s equilibrium is reached faster than in MD,
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along a path that is generally roughly similar to a typical MD
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simulation (but not necessarily so). See <a class="reference internal" href="#bal"><span>(Bal)</span></a> for details.</p>
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<p>Each step, all atoms in the selected group are displaced using the
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stochastic tfMC algorithm, which is designed to sample the canonical
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(NVT) ensemble at the temperature <em>Temp</em>. Although tfMC is a Monte
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Carlo algorithm and thus strictly speaking does not perform time
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integration, it is similar in the sense that it uses the forces on all
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atoms in order to update their positions. Therefore, it is implemented
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as a time integration fix, and no other fixes of this type (such as
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<a class="reference internal" href="fix_nve.html"><em>fix nve</em></a>) should be used at the same time. Because
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velocities do not play a role in this kind of Monte Carlo simulations,
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instantaneous temperatures as calculated by <a class="reference internal" href="compute_temp.html"><em>temperature computes</em></a> or <a class="reference internal" href="thermo_style.html"><em>thermodynamic output</em></a> have no meaning: the only relevant
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temperature is the sampling temperature <em>Temp</em>. Similarly, performing
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tfMC simulations does not require setting a <a class="reference internal" href="timestep.html"><em>timestep</em></a>
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and the <a class="reference internal" href="thermo_style.html"><em>simulated time</em></a> as calculated by LAMMPS is
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meaningless.</p>
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<p>The critical parameter determining the success of a tfMC simulation is
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<em>Delta</em>, the maximal displacement length of the lightest element in
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the system: the larger it is, the longer the effective time scale of
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the simulation will be (there is an approximately quadratic
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dependence). However, <em>Delta</em> must also be chosen sufficiently small
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in order to comply with detailed balance; in general values between 5
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and 10 % of the nearest neighbor distance are found to be a good
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choice. For a more extensive discussion with specific examples, please
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refer to <a class="reference internal" href="#bal"><span>(Bal)</span></a>, which also describes how the code calculates
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element-specific maximal displacements from <em>Delta</em>, based on the
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fourth root of their mass.</p>
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<p>Because of the uncorrelated movements of the atoms, the center-of-mass
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of the fix group will not necessarily be stationary, just like its
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orientation. When the <em>com</em> keyword is used, all atom positions will
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be shifted (after every tfMC iteration) in order to fix the position
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of the center-of-mass along the included directions, by setting the
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corresponding flag to 1. The <em>rot</em> keyword does the same for the
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rotational component of the tfMC displacements after every iteration.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">the <em>com</em> and <em>rot</em> keywords should not be used if an external
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force is acting on the specified fix group, along the included
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directions. This can be either a true external force (e.g. through
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<a class="reference internal" href="fix_wall.html"><em>fix wall</em></a>) or forces due to the interaction with atoms
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not included in the fix group. This is because in such cases,
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translations or rotations of the fix group could be induced by these
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external forces, and removing them will lead to a violation of
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detailed balance.</p>
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</div>
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</div>
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<hr class="docutils" />
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>.</p>
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<p>None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options are relevant to this
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fix.</p>
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<p>This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>This fix is part of the MC package. It is only enabled if LAMMPS was
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built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>This fix is not compatible with <a class="reference internal" href="fix_shake.html"><em>fix shake</em></a>.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="fix_gcmc.html"><em>fix gcmc</em></a>, <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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<p>The option default is com = 0 0 0</p>
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<hr class="docutils" />
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<p id="bal"><strong>(Bal)</strong> K. M Bal and E. C. Neyts, J. Chem. Phys. 141, 204104 (2014).</p>
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<p id="mees"><strong>(Mees)</strong> M. J. Mees, G. Pourtois, E. C. Neyts, B. J. Thijsse, and
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A. Stesmans, Phys. Rev. B 85, 134301 (2012).</p>
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<p id="neyts"><strong>(Neyts)</strong> E. C. Neyts and A. Bogaerts, Theor. Chem. Acc. 132, 1320
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(2013).</p>
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