# espressopp.integrator.BerendsenBarostat¶

This is the Berendsen barostat implementation according to the original paper [Berendsen84]. If Berendsen barostat is defined (as a property of integrator) then at the each run the system size and the particle coordinates will be scaled by scaling parameter $$\mu$$ according to the formula:

$\mu = [1 - \Delta t/\tau (P_{0} - P)]^{1/3}$

where $$\Delta t$$ - integration timestep, $$\tau$$ - time parameter (coupling parameter), $$P_{0}$$ - external pressure and $$P$$ - instantaneous pressure.

Example:

>>> berendsenP = espressopp.integrator.BerendsenBarostat(system)
>>> berendsenP.tau = 0.1
>>> berendsenP.pressure = 1.0


!IMPORTANT In order to run npt simulation one should separately define thermostat as well (e.g. BerendsenThermostat).

Definition:

In order to define the Berendsen barostat

>>> berendsenP = espressopp.integrator.BerendsenBarostat(system)


one should have the System defined.

Properties:

• berendsenP.tau

The property ‘tau’ defines the time parameter $$\tau$$.

• berendsenP.pressure

The property ‘pressure’ defines the external pressure $$P_{0}$$.

Setting the integration property:

>>> integrator.addExtension(berendsenP)


It will define Berendsen barostat as a property of integrator.

One more example:

>>> berendsen_barostat = espressopp.integrator.BerendsenBarostat(system)
>>> berendsen_barostat.tau = 10.0
>>> berendsen_barostat.pressure = 3.5


Canceling the barostat:

If one do not need the pressure regulation in system anymore or need to switch the ensamble or whatever :)

>>> # define barostat with parameters
>>> berendsen = espressopp.integrator.BerendsenBarostat(system)
>>> berendsen.tau = 0.8
>>> berendsen.pressure = 15.0
>>> ...
>>> # some runs
>>> ...
>>> # disconnect Berendsen barostat
>>> berendsen.disconnect()
>>> # the next runs will not include the system size and particle coordinates scaling


Connecting the barostat back after the disconnection

>>> berendsen.connect()


References:

 [Berendsen84] Berendsen et al., J. Chem. Phys., 81, 1984, p. 3684
espressopp.integrator.BerendsenBarostat(system)
Parameters: system –