[gmx-users] diverging temperature with pressure coupling

[David van der Spoel]

On 2011-04-18 11.22, Dommert Florian wrote:
> Hello,
>
> oh this nasty ILs ;) I am currently investigating a similar problem,
> that deals with PR-coupling and my impression is that Leap-Frog is the
> problem and not PR. Currently I am running simulations with md-vv and
> MTTK to verify my ideas. On the other hand I am using two tc_groups for
> my ILs due their difference in size. Unfortunately you have not written
> which IL you deal with, but thinking about long chain ILs like
> [BMIM][Cl], or [BMIM][PF6], the degrees of freedom for the different
> molecules differ strongly. Here the big question is why does the
> barostat influence the thermostat. It seems one has to be very very
> careful, when choosing the coupling parameter for PR. I did a bunch of
> simulations just varying the coupling parameter and obtained large
> differences in the dynamics of my system, while static properties like
> RDF, density of mass, and pressure look quite fine. For example if I
> calculate the conductivity including correlation effects the values
> differ from 2 S/m up to 12 S/m. But so far I also have no idea why ???
> I am really happy when I finally got my results with vv and if you are
> interested I can report back to the list or off-list, just let me know.
>
> On the other hand I do not know if the force field could be a problem.
> PR changes the box size according to a Lagrangian equation of motion in
> contrast to Berendsen, which just does size rescaling. I do not know how
> sensible this equations of motions are in respect to the motion of the
> atoms. Perhaps one can try to perform simulation with a higher accuracy
> for the electrostatic interactions. I realized that the standard PME
> settings are quite poor for ILs. Perhaps the Berendsen pressure coupling
> can deal with this, because it forces the box to scale. PR follows its
> equations of motions and PERHAPS introduces artefacts if the integration
> of atomic trajectories is too inaccurate.
>
> So my PME settings are like this:
>
> rcut=1.3 ( here the force field tells you the number )
> pme_order=6
> fourierspacing=0.05
> rtol=1e-8
>
> This are just approximate values, exact numbers can be calculated by
> g_pme_error for your system.
>
Florian, these PME values are scary! Are you running in double precision 
too, otherwise I guess it will be difficult to get the errors as low as 
you want them.

As regards the Temperature diversion (no pun intended), this is a known 
issue also with Berendsen T,P coupling. The reason for this is lack of 
interaction between the different subsystems. It is interesting to hear 
the PR makes it worse though, because of the better ensemble properties.

If this also has an effect on dynamic properties all of us are in even 
more trouble. I recently had problems reproducing dielectric constants. 
I will now  redo my simulation with Berendsen P-coupling instead of PR, 
even though grompp tells me not to!

> Cheers,
>
> Flo
>
>
> On Sun, 2011-04-17 at 03:10 -0400, Roland Schulz wrote:
>> Forwarding this email from my group colleague:
>>
>>
>> Dear Gromacs users,
>>
>>
>>
>> I am trying to simulate a cellulose fiber in an ionic liquid solution
>> in the NPT ensemble.  During the simulation, the entire system is
>> coupled to a thermostat.  Yet, I observe an inhomogeneous temperature
>> distribution throughout my system (hot-solvent/cold-solute) when I use
>> Parrinello-Rahman pressure coupling but NOT when I employ Berendsen
>> pressure coupling.  I have tested velocity-rescaling and the
>> Nose-Hoover scheme to keep the temperature constant and in both cases
>> Parrinello-Rahman pressure coupling seems to cause the solute’s
>> temperature to become significantly lower than the solvent’s (to
>> decompose temperatures, I am using “mdrun -rerun” with a run input
>> that defines tc_grps separately).
>>
>>
>>
>> I was wondering whether there were any known algorithmic reasons for
>> this unphysical temperature gradient when using Parrinello-Rahman
>> pressure coupling.
>>
>> Thank you.
>>
>> Barmak
>>
>>
>> Comment from me: The effect is large. The ionic liquid is 5 degrees
>> higher and the cellulose is 50 degrees lower (after 50ps, after that
>> it stays constant). With Berendsen pressure both parts fluctuate
>> around the same target temperature (as one would expect). Any reason
>> why one doesn't get the correct temperature with rerun? Or is their a
>> better way to get the temperature for different groups(for a
>> simulation with just one tc-group)? Any reason why Parrinello-Rahman
>> pressure coupling would have this effect on the temperature?
>>
>>
>> Roland
>>
>>
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>
>


-- 
David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se