forked from lijiext/lammps
177 lines
7.5 KiB
HTML
177 lines
7.5 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 addforce command
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</H3>
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<H3>fix addforce/cuda 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 addforce fx fy fz keyword value ...
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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>addforce = style name of this fix command
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<LI>fx,fy,fz = force component values (force units)
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<PRE> any of fx,fy,fz can be a variable (see below)
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</PRE>
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<LI>zero or more keyword/value pairs may be appended to args
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<LI>keyword = <I>region</I> or <I>energy</I>
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<PRE> <I>region</I> value = region-ID
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region-ID = ID of region atoms must be in to have added force
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<I>energy</I> value = v_name
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v_name = variable with name that calculates the potential energy of each atom in the added force field
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</PRE>
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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 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
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</PRE>
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<P><B>Description:</B>
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</P>
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<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
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atoms in a simulation, such as for a simulation of Poiseuille flow in
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a channel.
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</P>
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<P>Any of the 3 quantities defining the force components can be specified
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as an equal-style or atom-style <A HREF = "variable.html">variable</A>, namely <I>fx</I>,
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<I>fy</I>, <I>fz</I>. 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 used to determine the
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force component.
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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 force 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 force
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field with optional time-dependence as well.
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</P>
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<P>If the <I>region</I> keyword is used, the atom must also be in the
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specified geometric <A HREF = "region.html">region</A> in order to have force added
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to it.
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</P>
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<HR>
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<P>Adding a force to atoms implies a change in their potential energy as
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they move due to the applied force field. For dynamics via the "run"
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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).
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</P>
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<P>The <I>energy</I> 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
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constants and not as variables. This is because LAMMPS can compute
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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.
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</P>
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<P>The <I>energy</I> 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 HREF = "run.html">run</A> command. If the keyword is not used, LAMMPS will set
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the energy to 0.0, which is typically fine for dynamics.
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</P>
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<P>The <I>energy</I> 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
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atom-style <A HREF = "variable.html">variable</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 <I>fx</I>, <I>fy</I>, <I>fz</I>, the energy variable is specified as v_name,
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where name is the variable name.
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</P>
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<P>Note that when the <I>energy</I> 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 HREF = "variable.html">variable</A> is consistent with the force
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
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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
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converge properly.
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</P>
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<HR>
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<P>Styles with a <I>cuda</I> suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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<A HREF = "Section_accelerate.html">Section_accelerate</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.
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</P>
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<P>These accelerated styles are part of the USER-CUDA package. They are
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only enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_6">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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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.
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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 on the group of
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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".
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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_setforce.html">fix setforce</A>, <A HREF = "fix_aveforce.html">fix aveforce</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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