[gmx-users] Effect of pressure coupling time constant on equilibrium densities

[Miguel Caro]

Dear all,

I am doing some NPT calculations for simple liquids and liquid mixtures.
I have noticed that for water I can use small time constants for the
pressure coupling (with the Berendsen barostat). However, for
dimethylformamide, which is a larger molecule, my system "blows up" and
I need to increase the tau_p value.

As I understand it, Berendsen does not correctly reproduce the pressure
fluctuations that correspond to a canonical ensemble, however it is a
quick and easy way to equilibrate a system's pressure. As a matter of
fact, I had to switch to Berendsen after I encountered some problems
with more sophisticated barostats. I only use Berendsen to obtain the
right average density of my system, after which my production
calculations are carried out within the NVT ensemble.

If I understand it correctly, a small pressure time constant allows
quicker (but also more abrupt and "blowing-up-prone") pressure
equilibration than a large one, therefore having to increase this
constant is suboptimal. My question is the following: how does the
choice of pressure time constant for the Berendsen barostat affect (if
at all) the *average equilibrium pressure* in a Gromacs simulation (as
opposed to the *rate of convergence* or the *amplitude of pressure
fluctuations*)?

I would also like to know whether I should expect Berendsen to yield an
incorrect average density or only incorrect pressure fluctuations.

Many thanks in advance,
Miguel
-- 
*Dr. Miguel Caro*
/Postdoctoral researcher/
Department of Electrical Engineering and Automation,
and COMP Centre of Excellence in Computational Nanoscience
Aalto University, Finland
Personal email: *mcaroba at gmail.com*
Work: *miguel.caro at aalto.fi*
Website: http://mcaroba.dyndns.org