forked from lijiext/lammps
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169 lines
7.0 KiB
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>fix tfmc command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID tfmc Delta Temp seed keyword value
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>tfmc = style name of this fix command
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<LI>Delta = maximal displacement length (distance units)
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<LI>Temp = imposed temperature of the system
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<LI>seed = random number seed (positive integer)
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<LI>zero or more keyword/arg pairs may be appended
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<LI>keyword = <I>com</I> or <I>rot</I>
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<PRE> <I>com</I> args = xflag yflag zflag
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xflag,yflag,zflag = 0/1 to exclude/include each dimension
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<I>rot</I> args = none
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<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>
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<P><B>Description:</B>
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</P>
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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 HREF = "#Mees">(Mees)</A> and <A HREF = "#Bal">(Bal)</A>.
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</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 HREF = "#Neyts">(Neyts)</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 HREF = "jump.html">loop</A>.)
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</P>
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<P>The initial version of tfMC algorithm in <A HREF = "#Mees">(Mees)</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 <I>Delta</I> 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 HREF = "#Bal">(Bal)</A> for details.
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</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 <I>Temp</I>. 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 HREF = "fix_nve.html">fix nve</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 HREF = "compute_temp.html">temperature
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computes</A> or <A HREF = "thermo_style.html">thermodynamic
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output</A> have no meaning: the only relevant
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temperature is the sampling temperature <I>Temp</I>. Similarly, performing
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tfMC simulations does not require setting a <A HREF = "timestep.html">timestep</A>
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and the <A HREF = "thermo_style.html">simulated time</A> as calculated by LAMMPS is
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meaningless.
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</P>
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<P>The critical parameter determining the success of a tfMC simulation is
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<I>Delta</I>, 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, <I>Delta</I> 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 HREF = "#Bal">(Bal)</A>, which also describes how the code calculates
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element-specific maximal displacements from <I>Delta</I>, based on the
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fourth root of their mass.
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</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 <I>com</I> 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 <I>rot</I> keyword does the same for the
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rotational component of the tfMC displacements after every iteration.
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</P>
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<P>IMPORTANT NOTE: the <I>com</I> and <I>rot</I> keywords should not be used if an
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external force is acting on the specified fix group, along the
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included directions. This can be either a true external force (e.g.
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through <A HREF = "fix_wall.html">fix wall</A>) or forces due to the interaction
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with atoms not included in the fix group. This is because in such
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cases, translations or rotations of the fix group could be induced by
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these external forces, and removing them will lead to a violation of
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detailed balance.
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</P>
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<HR>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>.
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</P>
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<P>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
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fix.
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</P>
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<P>This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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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 HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>This fix is not compatible with <A HREF = "fix_shake.html">fix shake</A>.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_gcmc.html">fix gcmc</A>, <A HREF = "fix_nh.html">fix nvt</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option default is com = 0 0 0
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</P>
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<HR>
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<A NAME = "Bal"></A>
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<P><B>(Bal)</B> K. M Bal and E. C. Neyts, J. Chem. Phys. 141, 204104 (2014).
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</P>
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<A NAME = "Mees"></A>
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<P><B>(Mees)</B> M. J. Mees, G. Pourtois, E. C. Neyts, B. J. Thijsse, and
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A. Stesmans, Phys. Rev. B 85, 134301 (2012).
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</P>
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<A NAME = "Neyts"></A>
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<P><B>(Neyts)</B> E. C. Neyts and A. Bogaerts, Theor. Chem. Acc. 132, 1320
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(2013).
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</P>
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</HTML>
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