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< title > fix efield command — LAMMPS documentation< / title >
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< li > fix efield command< / li >
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< div class = "section" id = "fix-efield-command" >
< span id = "index-0" > < / span > < h1 > fix efield command< a class = "headerlink" href = "#fix-efield-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 efield ex ey ez 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 > efield = style name of this fix command< / li >
< li > ex,ey,ez = E-field component values (electric field units)< / li >
< li > any of ex,ey,ez 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 > or < em > energy< / 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
< em > energy< / em > value = v_name
v_name = variable with name that calculates the potential energy of each atom in the added E-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 external-field efield 1.0 0.0 0.0
fix kick external-field efield 0.0 0.0 v_oscillate
< / 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 a force F = qE to each charged atom in the group due to an
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external electric field being applied to the system. If the system
contains point-dipoles, also add a torque on the dipoles due to the
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external electric field.< / p >
< p > For charges, any of the 3 quantities defining the E-field components
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can be specified as an equal-style or atom-style
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< a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > , namely < em > ex< / em > , < em > ey< / em > , < em > ez< / em > . 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 used to determine the E-field component.< / p >
< p > For point-dipoles, equal-style variables can be used, but atom-style
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variables are not currently supported, since they imply a spatial
gradient in the electric field which means additional terms with
gradients of the field are required for the force and torque on
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dipoles.< / 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 E-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 E-field
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with optional time-dependence as well.< / 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 or torque to atoms implies a change in their potential
energy as they move or rotate due to the applied E-field.< / p >
< p > For dynamics via the “ run” command, this energy can be optionally
added to the system’ s potential energy for thermodynamic output (see
below). For energy minimization via the “ minimize” command, this
energy must be added to the system’ s potential energy to formulate a
self-consistent minimization problem (see below).< / p >
< p > The < em > energy< / em > keyword is not allowed if the added field is a constant
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vector (ex,ey,ez), with all components defined as numeric constants
and not as variables. This is because LAMMPS can compute the energy
for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey
+ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles
the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey +
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muz*ez).< / 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 for charged particles. It is not required for
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point-dipoles, but a warning is issued since the minimizer in LAMMPS
does not rotate dipoles, so you should not expect to be able to
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minimize the orientation of dipoles in an applied electric field.< / p >
< p > The < em > energy< / em > 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 energy of each
atom as function of its position. Like variables used for < em > ex< / em > , < em > ey< / em > ,
< em > ez< / 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
due to the electric field were a spring-like F = kx, then the energy
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formula should be E = -0.5kx^2. If you don’ t do this correctly, the
minimization will not converge properly.< / 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 due to
the electric field 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
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can include the forces added by this fix in a consistent manner.
I.e. there is a decrease in potential energy when atoms move in the
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direction of the added force due to the electric field.< / 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 added to the
group of atoms. The scalar and vector values calculated by this fix
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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 >
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< div class = "admonition note" >
< p class = "first admonition-title" > Note< / p >
< p class = "last" > If you want the fictitious potential energy associated 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 >
< p > This fix is part of the MISC 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 > Making LAMMPS< / span > < / a > section for more info.< / p >
< / 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_addforce.html" > < em > fix addforce< / em > < / a > < / p >
< p > < strong > Default:< / strong > none< / p >
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