we use REMD a lot but have never tried combining it with RESPA so this
could indeed be a bug.
On Sat, Jan 30, 2021 at 10:01 AM David A Case <david.case.rutgers.edu>
wrote:
> On Sat, Jan 30, 2021, Charo del Genio wrote:
> >
> >I strongly suspect that this behaviour is due to the REMD engine
> computing the energies at step numbers that are not multiples of nrespa,
> thereby getting meaningless results.
> >So, is this a bug, or is RESPA not supposed to be used in REMD runs?
>
> Thanks for your investigation here. It sounds like a bug.
>
> But there is a bigger point worth discussing here. Is RESPA really worth
> it in Amber simulations, whether or not they use replica exchange? Amber
> uses the "original" RESPA idea, which is know to have stability problems
> for outer time steps somewhere around 8 fs (for the types of systems we
> simulate). I think most Amber developers use hydrogen mass repartitioning
> and SHAKE, which allows a 4 fs time step. In that circumstance, even an
> nrespa of 2 might be dangerous, and in any event would provide only modest
> speedup. So, I guessing that the combination of REMD + RESPA has not been
> tested much.
>
> (REMD people: please chime in here!)
>
> So: we should either fix the REMD problem, or at least prevent people from
> using REMD + RESPA (assuming that your analysis is indeed correct). You
> could see how much of a performance hit you get by turning off RESPA --
> that
> might help us decide what course to take.
>
> As an aside: there are very promising "isokinetic" integrators that can
> avoid the resonance problems with RESPA, and allow quite long outer time
> steps. We might want to consider implementing those. But the code
> complexity can be hard: the forces in the outer time step have to be both
> smoothly varying and expensive to compute. (The long-range Lennard-Jones
> contributions come to mind here, but other codes concentrate on splitting
> "direct" non-bonded interactions into buckets, to reduce the pair-list size
> for the inner time steps.)
>
> ...thanks...dac
>
>
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Received on Sat Jan 30 2021 - 07:30:03 PST
