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vregime.in file β€” Cell parameter manipulation during MD

The vregime.in file is an optional input that allows controlled manipulation of cell parameters during an MD simulation.

Its structure and usage are closely analogous to the tregime.in file, but instead of temperatures it defines volume, lattice, or angle deformation regimes. This makes it especially useful for mechanical loading, strain-controlled simulations, and density equilibration of condensed-phase systems.

When vregime.in is provided, ReaxFF generates an additional output file:

  • fort.77 β€” records the targeted and applied cell-parameter changes over time

Purpose of vregime.in

Typical applications include:

  • Applying uniaxial or multiaxial strain
  • Crack propagation simulations
  • Mechanical response studies
  • Driving amorphous materials toward correct density
  • Controlled lattice expansion or compression

General properties

  • Format-free: spacing and column alignment are flexible
  • Stage-based: each line defines a new deformation stage
  • Multi-parameter: multiple cell parameters can be modified simultaneously
  • Optional rescaling: atomic coordinates may or may not follow the cell change

File format

Each non-comment line defines one volume (cell) regime stage.

Comment lines begin with #.

Column definition (conceptual)

start  #V  type1  change/it  rescale  [type2  change/it  rescale] ...

Where:

  • start β€” MD iteration at which this regime begins
  • #V β€” number of cell-parameter modifications in this stage
  • type β€” cell parameter to modify (a, b, c, alfa, beta, gamma)
  • change/it β€” incremental change applied per MD iteration
  • rescale β€” whether atomic coordinates are rescaled (y or n)

Additional parameter blocks may follow on the same line.

Supported cell parameters

Keyword Meaning
a Cell length a
b Cell length b
c Cell length c
alfa Cell angle Ξ±
beta Cell angle Ξ²
gamma Cell angle Ξ³

Example 2.17: vregime.in input file

The following example applies staged lattice and angle deformations.

# Volume regimes
# start  #V  type1  change/it  rescale   type2  change/it  rescale

0000     2   alfa   0.050000   y         beta  -0.05      y
0100     2   beta   0.050000   y         alfa  -0.05      y
0200     2   a      0.010000   y         b     -0.010     y
0300     2   a     -0.010000   y         b      0.010     y
0400     4   a     -0.010000   y         alfa   0.050     y        b   0.01   y   beta 0.05 y

Interpretation of the example

Stage 1 (iteration 0 β†’)

  • Increase Ξ± angle
  • Decrease Ξ² angle
  • Atomic coordinates are rescaled with the cell

Stage 2 (iteration 100 β†’)

  • Increase Ξ²
  • Decrease Ξ±
  • Coordinates rescaled

Stage 3 (iteration 200 β†’)

  • Increase lattice constant a
  • Decrease lattice constant b
  • Coordinates rescaled

Stage 4 (iteration 300 β†’)

  • Reverse the previous a/b strain

Stage 5 (iteration 400 β†’)

  • Apply simultaneous strain to:
  • a
  • b
  • Ξ±
  • Ξ²
  • Coordinates rescaled for all changes

Rescaling behavior

The rescale flag controls how strain is applied:

Value Effect
y Atom coordinates are rescaled with the cell
n Only cell parameters change; atoms remain fixed

Using rescale = n effectively concentrates strain at the periodic boundaries, which can be useful for fracture or interface studies.

Output behavior

  • ReaxFF writes deformation progress to fort.77
  • Each MD step records the updated target and applied cell parameters
  • Useful for post-processing strain–stress relationships

Typical use cases

  • Crack propagation under controlled strain
  • Mechanical testing of crystals and amorphous solids
  • Density relaxation of glasses
  • Anisotropic lattice deformation studies

Summary

  • vregime.in enables dynamic cell manipulation
  • Closely parallels tregime.in, but for volume and lattice parameters
  • Supports multi-parameter, staged deformation
  • Essential for mechanical and strain-driven ReaxFF simulations