[gmx-users] strange temperature coupling problem

Nickle Fan nicklefan at gmail.com
Fri Jan 13 06:34:14 CET 2006

> 3000ps is an extremely large coupling constant to want to use.
> In any case, "Simple" is wrong about the frequency of coupling. This
> coupling constant is a parameter in an algorithm that is applied each
> step that expresses how well-connected the group is to the thermal
> reservoir. It is not a time interval between applications of the
> algorithm.
> I suggest you both have a read of section 3.4.5 of the gromacs manual,
> where it will tell you that "The Berendsen algorithm is stable up to
> Delta T approx tau_t". Doubtless 3000ps is much larger than your
> simulation timestep, and the instability is not surprising!
> Mark
Thanks for your kind response, but I do not quite agree with your point. I
have used tau_t up to 2ps in my other simulations, and they works fine. In
fact, from version 3.1, it is allowed to not couple certain groups. I tried
that too, and the temperature of water still rises like crazy.

The manual say that the algorithm is stable upto \DeltaT = tau_t, and I
intepret that as "we can use tau_t as small as \Delta_t", i.e., it actually
pose a lower limit for the coupling constant, rather than an upper limit.
Correct if I am wrong.

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