Testing ReaxFF in LAMMPS

Simulation Setup

The simulation contains 30 atoms distributed inside a cubic simulation box of size

\[ 25 \times 25 \times 25 \ \text{Å}^3 \]

consisting of:

• 10 Hydrogen atoms

• 10 Carbon atoms

• 10 Oxygen atoms

Periodic boundary conditions were applied in all directions, and the simulation used the ReaxFF CHO potential with dynamic charge equilibration.

Initial atomic velocities were assigned at 1000 K using a Gaussian distribution, while the temperature was maintained using the NVT ensemble.

The simulation timestep was

\[ \Delta t = 0.25\ \text{fs} \]

and the total simulation length was

\[ 200000 \times 0.25\ \text{fs} = 50\ \text{ps} \]

Trajectory Visualization

Atomic trajectories were exported from LAMMPS and visualized using OVITO. The trajectory animation shows atoms moving throughout the simulation box while dynamically interacting under the reactive force field.

Species Analysis

One of the most useful features of ReaxFF is automatic species analysis. During the simulation, LAMMPS continuously tracks temporary molecular structures formed between atoms.

Initially, the system starts with isolated groups of H, C, and O atoms. However, after the simulation begins, the species analysis shows the rapid formation of molecular species such as:

• H₂

• O₂

• Isolated oxygen atoms

Interestingly, the carbon atoms largely remain grouped as a stable C₁₀ cluster throughout the simulation, while hydrogen and oxygen atoms reorganize into smaller molecular structures.

The species output also shows that the system rapidly stabilizes after the initial reactive stage. After approximately the first few hundred timesteps, the dominant molecular populations remain nearly constant, indicating that the system has reached a relatively stable reactive configuration at 1000 K.

All simulation files, analysis scripts, and visualization tools are available at GitHub.