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eregime.in file — Electric field control during MD

The eregime.in file is an optional input that allows ReaxFF to impose external electric fields during a simulation.

The applied electric field is fully coupled to the EEM charge model, meaning that the field polarizes the system self-consistently. This enables simulations of field-driven processes such as polarization, dielectric response, and electrochemical effects.

Purpose of eregime.in

With eregime.in, users can:

  • Turn electric fields on or off during a simulation
  • Apply fields along specific directions (x, y, z)
  • Define time-dependent field schedules
  • Apply multiple fields simultaneously (e.g., x and y)

When eregime.in is present, ReaxFF generates an additional output file:

  • fort.78 — electric field strength in each direction and the associated field energy term

Important physical limitations

⚠️ Periodic boundary caution
Electric fields do not function correctly if molecules cross a periodic boundary along the field direction. This leads to energy discontinuities.

Best practice - Use a large vacuum layer along the field direction - Ensure molecules remain within the same periodic image - Ideal for slab, surface, or capacitor-like geometries

General properties

  • Format-free input
  • Stage-based field definition
  • Supports multiple simultaneous fields
  • Compatible with EEM-based charge models

File format

Each non-comment line defines one electric-field regime stage.

Comment lines begin with #.

Column definition (conceptual)

start  #V  direction  magnitude(V/Å)  [direction  magnitude(V/Å)] ...

Where:

  • start — MD iteration at which this field stage begins
  • #V — number of electric-field components defined
  • direction — field direction (x, y, or z)
  • magnitude — field strength in V/Å

Additional direction–magnitude pairs may follow on the same line.

Example 2.18: eregime.in input file

# Electric field regimes
# start  #V  direction  Magnitude (V/Angstrom)

0000  1  x  0.010000
1000  1  x -0.010000
2000  1  y  0.010000
3000  1  y -0.010000
4000  2  x -0.010000  y -0.0100
5000  2  x  0.010000  y  0.0100

Interpretation of the example

Stage 1 (iteration 0 →)

  • Apply an electric field in the +x direction
  • Magnitude: 0.01 V/Å

Stage 2 (iteration 1000 →)

  • Reverse the x-field direction

Stage 3 (iteration 2000 →)

  • Apply a field in the +y direction

Stage 4 (iteration 3000 →)

  • Reverse the y-field direction

Stage 5 (iteration 4000 →)

  • Apply simultaneous fields in −x and −y directions

Stage 6 (iteration 5000 →)

  • Reverse both fields to +x and +y

Output behavior

  • ReaxFF writes electric-field data to fort.78
  • Output includes:
  • Field components in x, y, and z
  • Total electric-field energy contribution
  • Useful for post-processing polarization and dielectric response

Typical use cases

  • Polarization and dielectric response studies
  • Field-driven surface chemistry
  • Electrochemical interface simulations
  • Ferroelectric and piezoelectric materials modeling

Summary

  • eregime.in enables time-dependent electric fields
  • Fully coupled to EEM charge polarization
  • Supports multi-directional fields
  • Requires care with periodic boundaries
  • Essential for field-driven ReaxFF simulations