fix addforce command

Warning

GPU support for this command has not been tested and may not work as expected.

Syntax

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

  • ID, group-ID are documented in fix command

  • addforce = style name of this fix command

  • fx,fy,fz = force component values (force units)

any of fx,fy,fz can be a variable (see below)

  • zero or more keyword/value pairs may be appended to args

  • keyword = region or energy

region value = region-ID
  region-ID = ID of region atoms must be in to have added force
energy value = v_name
  v_name = variable with name that calculates the potential energy of each atom in the added force field

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 v_espace

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, namely fx, fy, fz. 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(s) used to determine the force component.

Equal-style variables can specify formulas with various mathematical functions, and include status_style 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 in order to have force added to it.

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 status output (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 Aspherix® 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 command. If the keyword is not used, Aspherix® will set the energy to 0.0, which is typically fine for dynamics.

Restart, fix_modify, output, run start/stop

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

This fix computes a global scalar and a global 3-vector of forces, which can be accessed by various output commands. 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 command.

Restrictions

none

Coarse-graining information:

Using coarsegraining in combination with this command might lead to different dynamics or system state and thus to inconsistencies.