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Berendsen barostat

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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):

\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}).