fix addforce command — LAMMPS documentation

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# fix addforce command

## 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 = every or region or energy

      every value = Nevery
        Nevery = add force every this many time steps
      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 \((f_x,f_y,f_z)\) to the corresponding component of the 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 three quantities defining the force components, namely \(f_x\), \(f_y\), and \(f_z\), can be specified as an equal-style or atom-style variable. 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 time step, and its value(s) will be used to determine the force component(s).

Equal-style variables can specify formulas with various mathematical functions and include thermo_style command keywords for the simulation box parameters, time step, 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 every keyword is used, the Nevery setting determines how often the forces are applied. The default value is 1, for every time step.

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 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 \(\vec F = (f_x,f_y,f_z)\), with all components defined as numeric constants and not as variables. This is because LAMMPS can compute the energy for each atom directly as

\[E = -\vec x \cdot \vec F = -(x f_x + y f_y + z f_z),\]

so that \(-\vec\nabla E = \vec F\).

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, 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 name of an atom-style variable which is used to compute the energy of each atom as function of its position. Like variables used for \(f_x\), \(f_y\), \(f_z\), the energy variable is specified as v_name, where name is the variable name.

Note that when the energy keyword is used during an energy minimization, you must ensure that the formula defined for the atom-style variable is consistent with the force variable formulas (i.e., that \(-\vec\nabla E = \vec F\)). For example, if the force were a spring-like, \(\vec F = -k\vec x\), then the energy formula should be \(E = \frac12 kx^2\). If you do not 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.

The fix_modify energy option is supported by this fix to add the potential energy inferred by the added force to the global potential energy of the system as part of thermodynamic output. The default setting for this fix is fix_modify energy no. Note that this energy is a fictitious quantity but is needed so that the minimize 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).

The fix_modify virial option is supported by this fix to add the contribution due to the added forces on atoms to both the global pressure and per-atom stress of the system via the compute pressure and compute stress/atom commands. The former can be accessed by thermodynamic output. The default setting for this fix is fix_modify virial no.

The fix_modify respa option is supported by this fix. This allows to set at which level of the r-RESPA integrator the fix is adding its forces. Default is the outermost level.

This fix computes a global scalar and a global three-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.

The forces due to this fix are imposed during an energy minimization, invoked by the minimize command. You should not specify force components with a variable that has time-dependence for use with a minimizer, since the minimizer increments the time step as the iteration count during the minimization.

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 energy option for this fix.

## Restrictions

> none

## Related commands

fix setforce, fix aveforce

## Default

The option default for the every keyword is every = 1.