102 lines
4.2 KiB
HTML
102 lines
4.2 KiB
HTML
<HTML>
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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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<HR>
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<H3>fix efield 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 efield ex ey ez
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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>efield = style name of this fix command
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<LI>ex,ey,ez = E-field component values (electric field units)
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<LI>any of ex,ey,ez can be a variable (see below)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix kick external-field efield 1.0 0.0 0.0
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fix kick external-field efield 0.0 0.0 v_oscillate
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</PRE>
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<P><B>Description:</B>
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</P>
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<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
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contains point-dipoles, also add a torque on the dipoles due to the
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external electric field.
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</P>
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<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 HREF = "variable.html">variable</A>, namely <I>ex</I>, <I>ey</I>, <I>ez</I>. If the value is a
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variable, it should be specified as v_name, where name is the variable
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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. For point-dipoles,
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equal-style variables can be used, but atom-style variables are not
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currently supported, since they imply a spatial gradient in the
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electric field which means additional terms with gradients of the
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field are required for the force and torque on dipoles.
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</P>
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<P>Equal-style variables can specify formulas with various mathematical
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functions, and include <A HREF = "thermo_style.html">thermo_style</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.
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</P>
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<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
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coordinates. Thus it is easy to specify a spatially-dependent E-field
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with optional time-dependence as well.
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</P>
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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>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the potential "energy" inferred by the added force to the
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system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>. This is a fictitious quantity but is
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needed so that the <A HREF = "minimize.html">minimize</A> command can include the
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forces added by this fix in a consistent manner. I.e. there is a
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decrease in potential energy when atoms move in the direction of the
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added force. The potential energy contribution of dipoles in the
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electric field is also included.
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</P>
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<P>This fix computes a global scalar and a global 3-vector of forces,
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which can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. The scalar is the potential
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energy discussed above. The vector is the total force added to the
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group of atoms. The scalar and vector values calculated by this fix
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are "extensive". The added torques for point-dipoles cannot currently
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be output in this manner.
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command.
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</P>
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<P>The forces due to this fix are imposed during an energy minimization,
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invoked by the <A HREF = "minimize.html">minimize</A> command. You should not
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specify force components with a variable that has time-dependence for
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use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.
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</P>
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<P>IMPORTANT NOTE: 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
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the system (the quantity being minimized), you MUST enable the
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<A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option for this fix.
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</P>
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<P><B>Restrictions:</B> none
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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_addforce.html">fix addforce</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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