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fix efield command :h3

[Syntax:]

fix ID group-ID efield ex ey ez :pre

ID, group-ID are documented in "fix"_fix.html command
efield = style name of this fix command
ex,ey,ez = E-field component values (electric field units)
any of ex,ey,ez can be a variable (see below) :ul

[Examples:]

fix kick external-field efield 1.0 0.0 0.0
fix kick external-field efield 0.0 0.0 v_oscillate :pre

[Description:]

Add a force F = qE to each charged atom in the group due to an
external electric field being applied to the system.

Any of the 3 quantities defining the E-field components can be
specified as an equal-style or atom-style "variable"_variable.html,
namely {ex}, {ey}, {ez}.  If the value is a 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 its value used to
determine the E-field component.

Equal-style variables can specify formulas with various mathematical
functions, and include "thermo_style"_thermo_style.html command
keywords for the simulation box parameters and timestep and elapsed
time.  Thus it is easy to specify a time-dependent E-field.

Atom-style variables can specify the same formulas as equal-style
variables but can also include per-atom values, such as atom
coordinates.  Thus it is easy to specify a spatially-dependent E-field
with optional time-dependence as well.

[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.  

The "fix_modify"_fix_modify.html {energy} 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 "thermodynamic
output"_thermo_style.html.  This is a fictitious quantity but is
needed so that the "minimize"_minimize.html command 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 direction of the
added force.

This fix computes a global scalar and a global 3-vector of forces,
which can be accessed by various "output
commands"_Section_howto.html#howto_15.  The scalar is the potential
energy discussed above.  The vector is the total force added to the
group of atoms.  The scalar and vector values calculated by this fix
are "extensive".

No parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.

The forces due to this fix are imposed during an energy minimization,
invoked by the "minimize"_minimize.html command.  You should not
specify force components with a variable that has time-dependence for
use with a minimizer, since the minimizer increments the timestep as
the iteration count during the minimization.

IMPORTANT NOTE: 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
"fix_modify"_fix_modify.html {energy} option for this fix.

[Restrictions:] none

[Related commands:]

"fix addforce"_fix_addforce.html

[Default:] none
