fix efield command — LAMMPS 15 May 2015 version documentation

# fix efield command

## Syntax

    fix ID group-ID efield ex ey ez keyword value ...

- ID, group-ID are documented in fix command
- efield = style name of this fix command
- ex,ey,ez = E-field component values (electric field units)
- any of ex,ey,ez 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 E-field

## Examples

    fix kick external-field efield 1.0 0.0 0.0
    fix kick external-field efield 0.0 0.0 v_oscillate

## Description

Add a force F = qE to each charged atom in the group due to an external electric field being applied to the system. If the system contains point-dipoles, also add a torque on the dipoles due to the external electric field.

For charges, any of the 3 quantities defining the E-field components can be specified as an equal-style or atom-style variable, namely ex, ey, ez. 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 E-field component.

For point-dipoles, equal-style variables can be used, but atom-style variables are not currently supported, since they imply a spatial gradient in the electric field which means additional terms with gradients of the field are required for the force and torque on dipoles.

Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent E-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 E-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 or torque to atoms implies a change in their potential energy as they move or rotate due to the applied E-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 field is a constant vector (ex,ey,ez), with all components defined as numeric constants and not as variables. This is because LAMMPS can compute the energy for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey + z*ez), so that -Grad(E) = F. Similarly for point-dipole particles the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey + muz*ez).

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 for charged particles. It is not required for point-dipoles, but a warning is issued since the minimizer in LAMMPS does not rotate dipoles, so you should not expect to be able to minimize the orientation of dipoles in an applied electric field.

The energy 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 ex, ey, ez, 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 insure that the formula defined for the atom-style variable is consistent with the force variable formulas, i.e. that -Grad(E) = F. For example, if the force due to the electric field 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.

The fix_modify energy option is supported by this fix to add the potential “energy” inferred by the added force due to the electric field to the system’s potential energy as part of thermodynamic output. This 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 due to the electric field.

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 added to the group of atoms. 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 timestep as the iteration count during the minimization.

Warning

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

This fix is part of the MISC package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

## Related commands

fix addforce

Default: none