Method and Theory

An initial system (and corresponding unit cell system) is supplied with box dimensions, \(a_i^0\), close to the equilibrium values. A LAMMPS simulation then integrates the atomic positions and velocities for a specified number of timesteps.

The calculation script allows for the use of different integration methods:

• nve integrates atomic positions without changing box dimensions or the system’s total energy.

• npt integrates atomic positions and applies Nose-Hoover style thermostat and barostat (equilibriate to specified T and P).

• nvt integrates atomic positions and applies Nose-Hoover style thermostat (equilibriate to specified T).

• nph integrates atomic positions and applies Nose-Hoover style barostat (equilibriate to specified P).

• nve+l integrates atomic positions and applies Langevin style thermostat (equilibriate to specified T).

• nph+l integrates atomic positions and applies Nose-Hoover style barostat and Langevin style thermostat (equilibriate to specified T and P).

Notes on the different control schemes:

• The Nose-Hoover barostat works by rescaling the box dimensions according to the measured system pressures.

• The Nose-Hoover thermostat works by rescaling the atomic velocities according to the measured system temperature (kinetic energy). Cannot be used with a temperature of 0 K.

• The Langevin thermostat works by modifying the forces on all atoms with both a dampener and a random temperature dependent fluctuation. Used at 0 K, only the force dampener is applied.

Notes on run parameter values. The proper time to reach equilibrium (equilsteps), and sample frequency to ensure uncorrelated measurements (thermosteps) is simulation dependent. They can be influenced by the potential, timestep size, crystal structure, integration method, presence of defects, etc. The default values of equilsteps = 20,000 and thermosteps = 100 are based on general rule-of-thumb estimates for bulk crystals and EAM potentials, and may or may not be adequate.