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< li > fix addforce command< / li >
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< a href = "http://lammps.sandia.gov" > Website< / a >
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< span id = "index-0" > < / span > < h1 > fix addforce command< a class = "headerlink" href = "#fix-addforce-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< / div >
< div class = "section" id = "fix-addforce-cuda-command" >
< h1 > fix addforce/cuda command< a class = "headerlink" href = "#fix-addforce-cuda-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 addforce fx fy fz keyword value ...
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < em > fix< / em > < / a > command< / li >
< li > addforce = style name of this fix command< / li >
< li > fx,fy,fz = force component values (force units)< / li >
< / ul >
< div class = "highlight-python" > < div class = "highlight" > < pre > any of fx,fy,fz can be a variable (see below)
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > zero or more keyword/value pairs may be appended to args< / li >
< li > keyword = < em > every< / em > or < em > region< / em > or < em > energy< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > every< / em > value = Nevery
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Nevery = add force every this many timesteps
< em > region< / em > value = region-ID
region-ID = ID of region atoms must be in to have added force
< em > energy< / em > value = v_name
v_name = variable with name that calculates the potential energy of each atom in the added force field
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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 kick flow addforce 1.0 0.0 0.0
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fix kick flow addforce 1.0 0.0 v_oscillate
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fix ff boundary addforce 0.0 0.0 v_push energy v_espace
< / pre > < / div >
< / div >
< / div >
< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > Add fx,fy,fz to the corresponding component of force for each atom in
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the group. This command can be used to give an additional push to
atoms in a simulation, such as for a simulation of Poiseuille flow in
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a channel.< / p >
< p > Any of the 3 quantities defining the force components can be specified
as an equal-style or atom-style < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > , namely < em > fx< / em > ,
< em > fy< / em > , < em > fz< / em > . If the value is a variable, it should be specified as
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v_name, where name is the variable name. In this case, the variable
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will be evaluated each timestep, and its value(s) used to determine
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the force component.< / 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 force field.< / 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 force
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field with optional time-dependence as well.< / p >
< p > If the < em > every< / em > keyword is used, the < em > Nevery< / em > setting determines how
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often the forces are applied. The default value is 1, for every
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timestep.< / p >
< p > If the < em > region< / em > keyword is used, the atom must also be in the
specified geometric < a class = "reference internal" href = "region.html" > < em > region< / em > < / a > in order to have force added
to it.< / p >
< hr class = "docutils" / >
< p > Adding a force to atoms implies a change in their potential energy as
they move due to the applied force field. For dynamics via the “ run”
command, this energy can be optionally added to the system’ s potential
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energy for thermodynamic output (see below). For energy minimization
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via the “ minimize” command, this energy must be added to the system’ s
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potential energy to formulate a self-consistent minimization problem
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(see below).< / p >
< p > The < em > energy< / em > keyword is not allowed if the added force is a constant
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vector F = (fx,fy,fz), with all components defined as numeric
constants and not as variables. This is because LAMMPS can compute
the energy for each atom directly as E = -x dot F = -(x*fx + y*fy +
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z*fz), so that -Grad(E) = F.< / p >
< p > The < em > energy< / em > keyword is optional if the added force is defined with
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one or more variables, and if you are performing dynamics via the
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< a class = "reference internal" href = "run.html" > < em > run< / em > < / a > command. If the keyword is not used, LAMMPS will set
the energy to 0.0, which is typically fine for dynamics.< / p >
< p > The < em > energy< / em > keyword is required if the added force is defined with
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one or more variables, and you are performing energy minimization via
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the “ minimize” command. The keyword specifies the name of an
atom-style < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > which is used to compute the
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energy of each atom as function of its position. Like variables used
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for < em > fx< / em > , < em > fy< / em > , < em > fz< / em > , the energy variable is specified as v_name,
where name is the variable name.< / p >
< p > Note that when the < em > energy< / em > keyword is used during an energy
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minimization, you must insure that the formula defined for the
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atom-style < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > is consistent with the force
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
were a spring-like F = kx, then the energy formula should be E =
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-0.5kx^2. If you don’ t do this correctly, the minimization will not
converge properly.< / p >
< hr class = "docutils" / >
< p > Styles with a < em > cuda< / em > suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
run faster, depending on your available hardware, as discussed in
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< a class = "reference internal" href = "Section_accelerate.html" > < em > Section_accelerate< / em > < / a > of the manual. The
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accelerated styles take the same arguments and should produce the same
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results, except for round-off and precision issues.< / p >
< p > These accelerated styles are part of the USER-CUDA package. They are
only enabled if LAMMPS was 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 >
< p > You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the < a class = "reference internal" href = "Section_start.html#start-7" > < span > -suffix command-line switch< / span > < / a > when you invoke LAMMPS, or you can
use the < a class = "reference internal" href = "suffix.html" > < em > suffix< / em > < / a > command in your input script.< / p >
< p > See < a class = "reference internal" href = "Section_accelerate.html" > < em > Section_accelerate< / em > < / a > of the manual for
more instructions on how to use the accelerated styles effectively.< / p >
< / div >
< hr class = "docutils" / >
< 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 > .< / p >
< p > The < a class = "reference internal" href = "fix_modify.html" > < em > fix_modify< / em > < / a > < em > energy< / em > option is supported by this
fix to add the potential “ energy” inferred by the added force to the
system’ s potential energy as part of < a class = "reference internal" href = "thermo_style.html" > < em > thermodynamic output< / em > < / a > . This is a fictitious quantity but is
needed so that the < a class = "reference internal" href = "minimize.html" > < em > minimize< / em > < / a > command can include the
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forces added by this fix in a consistent manner. I.e. there is a
decrease in potential energy when atoms move in the direction of the
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added force.< / p >
< p > This fix computes a global scalar and a global 3-vector of forces,
which can be accessed by various < a class = "reference internal" href = "Section_howto.html#howto-15" > < span > output commands< / span > < / a > . The scalar is the potential
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energy discussed above. The vector is the total force on the group of
atoms before the forces on individual atoms are changed by the fix.
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The scalar and vector values calculated by this fix are “ extensive” .< / 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.< / p >
< p > The forces due to this fix are imposed during an energy minimization,
invoked by the < a class = "reference internal" href = "minimize.html" > < em > minimize< / em > < / a > command. You should not
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specify force components with a variable that has time-dependence for
use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.< / p >
< div class = "admonition warning" >
< p class = "first admonition-title" > Warning< / p >
< p class = "last" > If you want the fictitious potential energy associated
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with the added forces to be included in the total potential energy of
the system (the quantity being minimized), you MUST enable the
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< a class = "reference internal" href = "fix_modify.html" > < em > fix_modify< / em > < / a > < em > energy< / em > option for this fix.< / p >
< / div >
< / 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 = "fix_setforce.html" > < em > fix setforce< / em > < / a > , < a class = "reference internal" href = "fix_aveforce.html" > < em > fix aveforce< / em > < / a > < / p >
< / div >
< div class = "section" id = "default" >
< h2 > Default< a class = "headerlink" href = "#default" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > The option default for the every keyword is every = 1.< / p >
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