Berendsen barostat

$\left. \frac{dP}{dt} \right\vert_{\mathrm {bath} } = \frac{P_0 - P}{\tau_P}$
where $P_0$ is the reference pressure, i.e. the pressure of the external pressure "bath", and $P$ is the instantaneous pressure. $\tau_P$ 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 $\mu$ is given by (Eq. 20):
$\mu = 1 - \frac{\kappa_T \Delta t}{3\tau_P} (P_0 -P)$
where $\kappa_T$ is the isothermal compressibility. The value of $\kappa_T$ 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 $\kappa_T = 4.6 \times 10^{-5} \mathrm{bar}^{-1}$).