fix efield command — LAMMPS documentation

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# fix efield command

# fix efield/tip4p command

## Syntax

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

- ID, group-ID are documented in fix command

- style = efield or efield/tip4p

- 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 or potential

      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
      potential value = v_name
        v_name = variable with name that calculates the electric potential 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
    fix kick external-field efield/tip4p 1.0 0.0 0.0

## Description

Add a force \(\vec{F} = q\vec{E}\) 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.

New in version 28Mar2023.

When the efield/tip4p style is used, the E-field will be applied to the position of the virtual charge site M of a TIP4P molecule instead of the oxygen position as it is defined by a corresponding TIP4P pair style. The forces on the M site due to the external field are projected on the oxygen and hydrogen atoms of the TIP4P molecules.

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

\[U_{efield} = -\vec{x} \cdot q\vec{E} = -q (x\cdot E_x + y\cdot E_y + z\cdot Ez),\]

so that \(-\nabla U_{efield} = \vec{F}\). Similarly for point-dipole particles the energy can be computed as

\[U_{efield} = -\vec{\mu} \cdot \vec{E} = -\mu_x\cdot E_x + \mu_y\cdot E_y + \mu_z\cdot E_z\]

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 (or potential keyword, described below) 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 ensure 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.

New in version 15Jun2023.

The potential keyword can be used as an alternative to the energy keyword to specify the name of an atom-style variable, which is used to compute the added electric potential to each atom as a function of its position. The variable should have units of electric field multiplied by distance (that is, in units real, the potential should be in volts). As with the energy keyword, the variable name is specified as “v_name”. The energy added by this fix is then calculated as the electric potential multiplied by charge.

The potential keyword is mainly intended for correct charge equilibration in simulations with fix qeq/reaxff, since with variable charges the electric potential can be known beforehand but the energy cannot. A small additional benefit is that the energy keyword requires an additional conversion to energy units which the potential keyword avoids. Thus, when the potential keyword is specified, the energy keyword must not be used. As with energy, the potential keyword is not allowed if the added field is a constant vector. The potential keyword is not supported by fix efield/tip4p.

## 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 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 due to the electric field.

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 adding its forces. Default is the outermost level.

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.

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

Fix style efield/tip4p is part of the EXTRA-FIX package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

Fix style efield/tip4p can only be used with tip4p pair styles.

## Related commands

fix addforce

## Default

none