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tregime.in file — Temperature regimes for MD simulations

The tregime.in file defines temperature regimes for NVT or NPT molecular dynamics (MD) simulations in ReaxFF.

It allows the system to be divided into multiple temperature zones, each of which can follow independent temperature schedules over different stages of the simulation. This enables controlled heating, cooling, and annealing protocols.

Purpose of tregime.in

When present, tregime.in allows you to:

  • Apply different temperatures to different atom groups
  • Heat or cool selected regions at controlled rates
  • Perform staged annealing simulations
  • Monitor zone-resolved temperature evolution

When tregime.in is used, ReaxFF generates an additional output file:

  • fort.75 — temperature information for each defined zone

File format

Each non-comment line in tregime.in defines one temperature stage.

Comment lines start with #.

Column definition

# Start  #Zones  At1  At2  Tset1  Tdamp1  dT1/dIt   At3  At4  Tset2  Tdamp2  dT2/dIt

Where:

  • Start — MD iteration at which this stage begins
  • #Zones — number of temperature zones
  • At1–At2 — atom index range for zone 1
  • Tset1 — target temperature for zone 1 (K)
  • Tdamp1 — temperature damping constant for zone 1
  • dT1/dIt — temperature change per MD iteration for zone 1 (K/iteration)
  • At3–At4, Tset2, Tdamp2, dT2/dIt — same quantities for zone 2

Additional zones follow the same pattern.

Example 2.12: tregime.in input file

# Start  #Zones  At1  At2  Tset1  Tdamp1  dT1/dIt  At3  At4  Tset2  Tdamp2  dT2/dIt
0        2       1    20   200.0  50.0    0.05     21   40   10.0   1.0     0.00
10000    2       1    20   700.0  50.0    0.00     21   40   10.0   1.0     0.00
20000    2       1    20   700.0  50.0   -0.10     21   40   10.0   1.0     0.00

Interpretation of the example

This example divides the system into two temperature zones:

  • Zone 1: atoms 1–20
  • Zone 2: atoms 21–40

Stage 1 (iteration 0–10,000)

  • Zone 1 is heated from 200 K to 700 K
  • Heating rate: +0.05 K/iteration
  • Zone 2 is held constant at 10 K

Stage 2 (iteration 10,000–20,000)

  • Zone 1 remains at 700 K
  • Zone 2 remains at 10 K

Stage 3 (iteration 20,000–end)

  • Zone 1 is cooled at −0.10 K/iteration
  • Zone 2 remains at 10 K

This setup performs a localized annealing protocol, where only part of the system is thermally driven.

General notes

  • There is no fixed limit on:
  • Number of temperature zones
  • Number of temperature stages
  • Temperature zones may overlap only if physically meaningful
  • tregime.in is compatible with both NVT and NPT simulations
  • The temperature evolution of each zone is written to fort.75

Typical use cases

  • Surface annealing while keeping bulk atoms cold
  • Thermal activation of reaction sites
  • Localized heating for diffusion studies
  • Multi-step heating–cooling schedules

tregime.in provides fine-grained thermal control and is a powerful tool for advanced ReaxFF MD workflows.