Berendsen Barostat

With the Berendsen barostat the system is made to obey the equation of motion

$${d{\cal P}\over dt} = (P_{\rm ext} - {\cal P})/\tau_P$$ (2.198)

Cell size variations

In the isotropic implementation, at each step the MD cell volume is scaled by by a factor $\eta$ and the coordinates, and cell vectors, by $\eta^{1/3}$ where

$$\eta = 1 -{ \beta\Delta t \over \tau_P} (P_{\rm ext} -{\cal P})$$ (2.199)

and $\beta$ is the isothermal compressibility of the system. The Berendesen thermostat is applied at the same time. In practice $\beta$ is a specified constant which DL_POLY_2 takes to be the isothermal compressibility of liquid water. The exact value is not critical to the algorithm as it relies on the ratio $\tau_P/\beta$. $\tau_P$ is specified by the user.

This algorithm is implemented in NPT_B1 with 4 or 5 iterations used to obtain self consistency in the $\mbox{$\underline{v}$}(t)$.

Cell size and shape variations

The extension of the isotropic algorithm to anisotropic cell variations is straightforward. The tensor $\underline{\underline{\bf\eta}}$ is defined by

$$\mbox{$\underline{\underline{\bf\eta}}$} = \mbox{$\underlin... ...erline{\bf 1}}$} - \mbox{$\underline{\underline{\bf\sigma}}$})$$ (2.200)

and the new cell vectors given by

$$\mbox{$\underline{\underline{\bf H}}$}(t +\Delta t) \leftarr... ...nderline{\bf H}}$}(t) \mbox{$\underline{\underline{\bf\eta}}$}$$ (2.201)

As in the isotropic case the Berendsen thermostat is applied simultaneously and 4 or 5 iterations are used to obtain convergence. The algorithm is implemented in NST_B0 (nonbonded systems) and NST_B1 (with bond constraints).