forked from lijiext/lammps
177 lines
7.4 KiB
HTML
177 lines
7.4 KiB
HTML
<HTML>
|
|
<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>
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
<H3>fix addforce command
|
|
</H3>
|
|
<H3>fix addforce/cuda command
|
|
</H3>
|
|
<P><B>Syntax:</B>
|
|
</P>
|
|
<PRE>fix ID group-ID addforce fx fy fz keyword value ...
|
|
</PRE>
|
|
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
|
|
|
|
<LI>addforce = style name of this fix command
|
|
|
|
<LI>fx,fy,fz = force component values (force units)
|
|
|
|
<LI>any of fx,fy,fz can be a variable (see below)
|
|
|
|
<LI>zero or more keyword/value pairs may be appended to args
|
|
|
|
<LI>keyword = <I>region</I> or <I>energy</I>
|
|
|
|
<PRE> <I>region</I> value = region-ID
|
|
region-ID = ID of region atoms must be in to have added force
|
|
<I>energy</I> value = v_name
|
|
v_name = variable with name that calculates the potential energy of each atom in the added force field
|
|
</PRE>
|
|
|
|
</UL>
|
|
<P><B>Examples:</B>
|
|
</P>
|
|
<PRE>fix kick flow addforce 1.0 0.0 0.0
|
|
fix kick flow addforce 1.0 0.0 v_oscillate
|
|
fix ff boundary addforce 0.0 0.0 v_push energy v_espace
|
|
</PRE>
|
|
<P><B>Description:</B>
|
|
</P>
|
|
<P>Add fx,fy,fz to the corresponding component of force for each atom in
|
|
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
|
|
a channel.
|
|
</P>
|
|
<P>Any of the 3 quantities defining the force components can be specified
|
|
as an equal-style or atom-style <A HREF = "variable.html">variable</A>, namely <I>fx</I>,
|
|
<I>fy</I>, <I>fz</I>. 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
|
|
force component.
|
|
</P>
|
|
<P>Equal-style variables can specify formulas with various mathematical
|
|
functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
|
|
keywords for the simulation box parameters and timestep and elapsed
|
|
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
|
|
variables but can also include per-atom values, such as atom
|
|
coordinates. Thus it is easy to specify a spatially-dependent force
|
|
field with optional time-dependence as well.
|
|
</P>
|
|
<P>If the <I>region</I> keyword is used, the atom must also be in the
|
|
specified geometric <A HREF = "region.html">region</A> in order to have force added
|
|
to it.
|
|
</P>
|
|
<HR>
|
|
|
|
<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
|
|
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 <I>energy</I> keyword is not allowed if the added force is a constant
|
|
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 +
|
|
z*fz), so that -Grad(E) = F.
|
|
</P>
|
|
<P>The <I>energy</I> keyword is optional if the added force is defined with
|
|
one or more variables, and if you are performing dynamics via the
|
|
<A HREF = "run.html">run</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 <I>energy</I> keyword is required if the added force is defined with
|
|
one or more variables, and you are performing energy minimization via
|
|
the "minimize" command. The keyword specifies the name of an
|
|
atom-style <A HREF = "variable.html">variable</A> which is used to compute the
|
|
energy of each atom as function of its position. Like variables used
|
|
for <I>fx</I>, <I>fy</I>, <I>fz</I>, the energy variable is specified as v_name,
|
|
where name is the variable name.
|
|
</P>
|
|
<P>Note that when the <I>energy</I> keyword is used during an energy
|
|
minimization, you must insure that the formula defined for the
|
|
atom-style <A HREF = "variable.html">variable</A> is consistent with the force
|
|
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 =
|
|
-0.5kx^2. If you don't do this correctly, the minimization will not
|
|
converge properly.
|
|
</P>
|
|
<HR>
|
|
|
|
<P>Styles with a <I>cuda</I> suffix are functionally the same as the
|
|
corresponding style without the suffix. They have been optimized to
|
|
run faster, depending on your available hardware, as discussed in
|
|
<A HREF = "Section_accelerate.html">this section</A> of the manual. The accelerated
|
|
styles take the same arguments and should produce the same 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 HREF = "Section_start.html#2_3">Making LAMMPS</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 HREF = "Section_start.html#2_6">-suffix command-line
|
|
switch</A> when you invoke LAMMPS, or you can use
|
|
the <A HREF = "suffix.html">suffix</A> command in your input script.
|
|
</P>
|
|
<P>See <A HREF = "Section_accelerate.html">this section</A> of the manual for more
|
|
instructions on how to use the accelerated styles effectively.
|
|
</P>
|
|
<HR>
|
|
|
|
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
|
|
</P>
|
|
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
|
|
files</A>.
|
|
</P>
|
|
<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> 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 HREF = "thermo_style.html">thermodynamic
|
|
output</A>. This is a fictitious quantity but is
|
|
needed so that the <A HREF = "minimize.html">minimize</A> 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.
|
|
</P>
|
|
<P>This fix computes a global scalar and a global 3-vector of forces,
|
|
which can be accessed by various <A HREF = "Section_howto.html#4_15">output
|
|
commands</A>. The scalar is the potential 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. The
|
|
scalar and vector values calculated by this fix are "extensive".
|
|
</P>
|
|
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
|
|
the <A HREF = "run.html">run</A> command.
|
|
</P>
|
|
<P>The forces due to this fix are imposed during an energy minimization,
|
|
invoked by the <A HREF = "minimize.html">minimize</A> 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.
|
|
</P>
|
|
<P>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
|
|
<A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option for this fix.
|
|
</P>
|
|
<P><B>Restrictions:</B> none
|
|
</P>
|
|
<P><B>Related commands:</B>
|
|
</P>
|
|
<P><A HREF = "fix_setforce.html">fix setforce</A>, <A HREF = "fix_aveforce.html">fix aveforce</A>
|
|
</P>
|
|
<P><B>Default:</B> none
|
|
</P>
|
|
</HTML>
|