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

[Mark Abraham]

Hi,

On Tue, Jun 21, 2016 at 4:47 PM Miguel Caro <miguel.caro at aalto.fi> wrote:

> 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.
>

The molecular size and the need for tau_p to be from any particular range
of values should have no relationship. I'd be looking at your equilibration
protocol, constraints settings and timestep.

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.
>

Fine, do that.


> 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*)?
>

It doesn't affect the equilibrium value, only the rate of approach to it
and the nature of the fluctuations around it.

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

The average is correct, given your model of reality.

Mark


> 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
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