Re: [AMBER] REMD exchange

From: Francesco Pietra <>
Date: Tue, 10 Sep 2013 15:45:20 +0200

>Assuming that I am interpreting your question correctly,

Yes, and the answer is very useful: I can assume a larger delta T than the
0.8K I reported. Provided that I will be able to calculate the exchange
frequency (so far unclear to me how; I can just change the number of
steps/run, I used 100, if increased to 1000 there is no more any exchange).

At any event, are >200 replicas (as needed with my system for 314-600K) a
reasonable prospect or one that is destined to result as technically
unfeasible? The literature is abundant of variants on small peptides.

Finally, is exhaustive MD equilibration an absolute need or it can just
help? So far I have not insisted in the equilibration at 314K (rmsd/frame
is still raising because - I suppose - of the flexibility of the portion to
model: clearly there are low energy barriers)


francesco pietra

On Tue, Sep 10, 2013 at 1:12 PM, Jason Swails <>wrote:

> On Tue, Sep 10, 2013 at 3:38 AM, Francesco Pietra <
> >wrote:
> > Hello:
> > I am progressing toward a major task of carrying out a pure T-remd
> > (parallel tempering) with a 34aa peptide in explicit water for a 314-600K
> > range. After C. Simmerling reports about implic media, I did not attempt
> > any simulation under implicit media.
> >
> > I started with 8 replicas and geom progressing temp, noticing, for the
> > 314-320K range, full exchange is obtained. deltaT is 0.8K.
> >
> > On increasing the nr of replicas and delta T on the above basis, thus 16
> > replicas for 314-326K, the exchange is no more so good. That is, the
> > highest and the lowest temp do not exchange with one another directly.
> >
> > My question is, do, for example, exchange of 16 with 15 only, and
> exchange
> > of 14 with 15, compensate? In other words, do "local exchanges"
> propagate?
> >
> > In this affair, I restrained the dihedrals for a two (starting) alpha
> > helical turns, which are well defined by X-ray diffraction. The rest of
> the
> > peptide does not clearly diffract at 100K, and I am attempting to
> simulate
> > how the conformation, or prevailing cluster of conformations, should be.
> I
> > did not apply any other restraint to this undefined part. Should the
> > chirality at the peptide bond be imposed even if the prevailing
> > conformation is unknown?
> >
> > Thanks so much for advice
> >
> Assuming that I am interpreting your question correctly, there is no need
> to achieve exchanges between the first and last temperature replica. In
> fact, if you are achieving exchanges between those two a high percentage of
> the time, then all of the replicas in between are unnecessary :). As long
> as adjacent replicas exchange frequently enough (about 20% of the time is
> good in my experience), then you will get good enough mixing in state space
> to make efficient use of REMD. (Replicas will be able to diffuse through
> state space from the lowest temperature to the highest _through_ the
> intermediate temperatures).
> The following website will give you a good set of starting temperatures
> between two replicas: (the NPT results are
> good enough for the NVT REMD that Amber runs). It may be worth running a
> quick simulation to verify that all exchange rates are approximately what
> you want them to be before starting major production.
> HTH,
> Jason
> --
> Jason M. Swails
> BioMaPS,
> Rutgers University
> Postdoctoral Researcher
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Received on Tue Sep 10 2013 - 07:00:02 PDT
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