# Berendsen barostat

The **Berendsen barostat** ^{[1]} is a method for controlling the pressure in a molecular dynamics simulation.
The Berendsen barostat adds an extra term to to the equations of motion which effects the pressure change (Eq. 12):

where is the reference pressure, i.e. the pressure of the external pressure "bath", and is the instantaneous pressure. is a time constant. Within this scheme the coordinates and the box sides are rescaled every step. Assuming the system is isotropic and within a cubic box the scaling factor is given by (Eq. 20):

where is the isothermal compressibility. The value of only has to be reasonable; for example, both DL POLY and GROMACS use the value of the compressibility of water (at 1 atm and 300K, leading to ).