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< title > fix heat command — LAMMPS 17 Dec 2015 documentation< / title >
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< li > fix heat command< / li >
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< span id = "index-0" > < / span > < h1 > fix heat command< a class = "headerlink" href = "#fix-heat-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > fix ID group-ID heat N eflux
< / pre > < / div >
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< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < em > fix< / em > < / a > command< / li >
< li > heat = style name of this fix command< / li >
< li > N = add/subtract heat every this many timesteps< / li >
< li > eflux = rate of heat addition or subtraction (energy/time units)< / li >
< li > eflux can be a variable (see below)< / li >
< li > zero or more keyword/value pairs may be appended to args< / li >
< li > keyword = < em > region< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > region< / em > value = region-ID
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region-ID = ID of region atoms must be in to have added force
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< / pre >
< / div >
< div class = "section" id = "examples" >
< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > fix 3 qin heat 1 1.0
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fix 3 qin heat 10 v_flux
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fix 4 qout heat 1 -1.0 region top
< / pre > < / div >
< / div >
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< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Add non-translational kinetic energy (heat) to a group of atoms in a
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manner that conserves their aggregate momentum. Two of these fixes
can be used to establish a temperature gradient across a simulation
domain by adding heat (energy) to one group of atoms (hot reservoir)
and subtracting heat from another (cold reservoir). E.g. a simulation
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sampling from the McDLT ensemble.< / p >
< p > If the < em > region< / em > keyword is used, the atom must be in both the group
and the specified geometric < a class = "reference internal" href = "region.html" > < em > region< / em > < / a > in order to have
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energy added or subtracted to it. If not specified, then the atoms in
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the group are affected wherever they may move to.< / p >
< p > Heat addition/subtraction is performed every N timesteps. The < em > eflux< / em >
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parameter can be specified as a numeric constant or as a variable (see
below). If it is a numeric constant or equal-style variable which
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evaluates to a scalar value, then the < em > eflux< / em > determines the change in
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aggregate energy of the entire group of atoms per unit time, e.g. in
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eV/psec for < a class = "reference internal" href = "units.html" > < em > metal units< / em > < / a > . In this case it is an
“ extensive” quantity, meaning its magnitude should be scaled with the
number of atoms in the group. Note that since < em > eflux< / em > has per-time
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units (i.e. it is a flux), this means that a larger value of N will
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add/subtract a larger amount of energy each time the fix is invoked.< / p >
< p > If < em > eflux< / em > is specified as an atom-style variable (see below), then
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the variable computes one value per atom. In this case, each value is
the energy flux for a single atom, again in units of energy per unit
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time. In this case, each value is an “ intensive” quantity, which need
not be scaled with the number of atoms in the group.< / p >
< p > As mentioned above, the < em > eflux< / em > parameter can be specified as an
equal-style or atom_style < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > . If the value is a
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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
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its value(s) used to determine the flux.< / p >
< p > Equal-style variables can specify formulas with various mathematical
functions, and include < a class = "reference internal" href = "thermo_style.html" > < em > thermo_style< / em > < / a > command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent flux.< / p >
< p > Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
coordinates. Thus it is easy to specify a spatially-dependent flux
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with optional time-dependence as well.< / p >
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< div class = "admonition note" >
< p class = "first admonition-title" > Note< / p >
< p class = "last" > If heat is subtracted from the system too aggressively so that
the group’ s kinetic energy would go to zero, or any individual atom’ s
kinetic energy would go to zero for the case where < em > eflux< / em > is an
atom-style variable, then LAMMPS will halt with an error message.< / p >
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< / div >
< p > Fix heat is different from a thermostat such as < a class = "reference internal" href = "fix_nh.html" > < em > fix nvt< / em > < / a >
or < a class = "reference internal" href = "fix_temp_rescale.html" > < em > fix temp/rescale< / em > < / a > in that energy is
added/subtracted continually. Thus if there isn’ t another mechanism
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in place to counterbalance this effect, the entire system will heat or
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cool continuously. You can use multiple heat fixes so that the net
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energy change is 0.0 or use < a class = "reference internal" href = "fix_viscous.html" > < em > fix viscous< / em > < / a > to drain
energy from the system.< / p >
< p > This fix does not change the coordinates of its atoms; it only scales
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their velocities. Thus you must still use an integration fix
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(e.g. < a class = "reference internal" href = "fix_nve.html" > < em > fix nve< / em > < / a > ) on the affected atoms. This fix should
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not normally be used on atoms that have their temperature controlled
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by another fix - e.g. < a class = "reference internal" href = "fix_nh.html" > < em > fix nvt< / em > < / a > or < a class = "reference internal" href = "fix_langevin.html" > < em > fix langevin< / em > < / a > fix.< / p >
< / div >
< div class = "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 = "Permalink to this headline" > ¶< / a > < / h2 >
< p > No information about this fix is written to < a class = "reference internal" href = "restart.html" > < em > binary restart files< / em > < / a > . None of the < a class = "reference internal" href = "fix_modify.html" > < em > fix_modify< / em > < / a > options
are relevant to this fix.< / p >
< p > This fix computes a global scalar which can be accessed by various
< a class = "reference internal" href = "Section_howto.html#howto-15" > < span > output commands< / span > < / a > . This scalar is the
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most recent value by which velocites were scaled. The scalar value
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calculated by this fix is “ intensive” . If < em > eflux< / em > is specified as
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an atom-style variable, this fix computes the average value by which
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the velocities were scaled for all of the atoms that had their
velocities scaled.< / 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" > < em > run< / em > < / a > command. This fix is not invoked during < a class = "reference internal" href = "minimize.html" > < em > energy minimization< / em > < / a > .< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< blockquote >
< div > none< / div > < / blockquote >
< / div >
< div class = "section" id = "related-commands" >
< h2 > Related commands< a class = "headerlink" href = "#related-commands" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > < a class = "reference internal" href = "compute_temp.html" > < em > compute temp< / em > < / a > , < a class = "reference internal" href = "compute_temp_region.html" > < em > compute temp/region< / em > < / a > < / p >
< p > < strong > Default:< / strong > none< / p >
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