Barostat

A barostat is a device or algorithm that maintains or regulates pressure within a system. In experimental settings, a barostat typically controls the pressure of a sample by adjusting the volume of the surrounding chamber—often via a piston or by managing a connected gas reservoir—to match a target pressure.

In computational science, a barostat refers to an algorithm that allows the simulation box to fluctuate in size and, in some cases, shape, to keep the external pressure at a specified value. This enables simulations to sample the NPT ensemble (constant number of particles, pressure, and temperature). Barostat operation is usually paired with a thermostat to control temperature and with either isotropic or anisotropic volume changes. Isotropic barostats scale the entire box uniformly, while anisotropic barostats permit independent changes to the box vectors, allowing the simulation cell to change shape.

Common barostat algorithms include:

- Berendsen barostat, a simple weak-coupling method used mainly for rapid equilibration, which does not generate correct pressure fluctuations for rigorous sampling.

- Parrinello-Rahman barostat, an extended-Lagrangian approach that allows anisotropic fluctuations and is suitable for solids and phase transitions.

- Martyna-Tuckerman-Tobias-Keffer (MTTK) barostat, a rigorous method often combined with Nosé-Hoover thermostats to provide proper NPT sampling.

- Monte Carlo barostats, which adjust volume through Metropolis criteria rather than through dynamical equations.

Barostats are essential for studying materials, liquids, and biomolecular systems under pressure, enabling analysis of properties such as compressibility, phase behavior, and pressure-dependent kinetics. The choice of barostat can influence sampling and dynamics, so selection should align with the system and research goals.