lammps/doc/fix_addforce.txt

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:line

fix addforce command :h3

[Syntax:]

fix ID group-ID addforce fx fy fz keyword value ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
addforce = style name of this fix command :l
fx,fy,fz = force component values (force units) :l
any of fx,fy,fz can be a variable (see below) :l
zero or more keyword/value pairs may be appended to args :l
keyword = {region}  :l
  {region} value = region-ID
    region-ID = ID of region atoms must be in to have added force
  {energy} value = variable-ID
    variable-ID = ID of variable that calculates the potential energy of each atom in the added force field :pre
:ule

[Examples:]

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 espace :pre

[Description:]

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.

Any of the 3 quantities defining the force components can be specified
as an equal-style or atom-style "variable"_variable.html, namely {fx},
{fy}, {fz}.  If the value is a variable, it should be specified as
v_ID, where ID is the variable ID.  In this case, the variable will be
evaluated each timestep, and its value used to determine the force
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 force 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 force
field with optional time-dependence as well.

If the {region} keyword is used, the atom must also be in the
specified geometric "region"_region.html in order to have force added
to it.

:line

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).

The {energy} 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.

The {energy} keyword is optional if the added force is defined with
one or more variables, and if you are performing dynamics via the
"run"_run.html command.  If the keyword is not used, LAMMPS will set
the energy to 0.0, which is typically fine for dynamics.

The {energy} 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 ID of an atom-style
"variable"_variable.html which is used to compute the energy of each
atom as function of its position.  Unlike variables used for {fx},
{fy}, {fz}, the energy variable is specified by ID only, without a
"v_" prefix.

Note that when the {energy} keyword is used during an energy
minimization, you must insure that the formula defined for the
atom-style "variable"_variable.html 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.

[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#4_15.  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".

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 setforce"_fix_setforce.html, "fix aveforce"_fix_aveforce.html

[Default:] none