Re: [AMBER-Developers] requirement for an even number of replicas in replica exchange?

From: Adrian Roitberg <>
Date: Wed, 25 Sep 2019 18:15:07 +0200

Just to add one potential issue, I am going to guess that the
calculations for the remlog files makes the implicit assumption of an
even number of replicas, so there is a certain symmetry on how it is
computed and when.

On 9/25/19 6:11 PM, koushik kasavajhala wrote:
> EXTERNAL EMAIL: Exercise caution with links and attachments.
> To add to what Carlos has said, the main problem might arise during
> mpi_send() and mpi_recv() calls. Currently, different variables are sent
> and received during REMD using the MPI protocols. I am not sure how these
> would work when odd number of replicas/processors are used. Maybe others
> might know how to overcome this. In terms of doing the exchange calculation
> itself, it should be trivial to perform with odd number of replicas.
> On Wed, Sep 25, 2019 at 11:38 AM Carlos Simmerling <
>> wrote:
>> I think conceptually it isn't hard. An easy way would be to alternate
>> having the first or last replica not involved in exchanges, leaving an even
>> number to pair for exchange. Right now the first/last attempt exchange on
>> alternate attempts, which usually fails, so having one of them do nothing
>> every other time would be similar. The bookkeeping would need to be updated
>> for things that assume even numbers, and selection of partners is a little
>> more complicated, but I think it should work without too much rewriting. It
>> was always easier to code with even numbers, and an extra replica didn't
>> seem to cost much so we did it that way.
>> Others might know of specific issues in the current code that would be
>> difficult to change...
>> carlos
>> On Wed, Sep 25, 2019, 10:12 AM David Case <> wrote:
>>> Does anyone have a feeling of how easy/hard it would be allow an odd
>>> number of replicas in (Hamiltonian) replica exchange?
>>> ...thx...dac
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Dr. Adrian E. Roitberg
University of Florida Research Foundation Professor
Department of Chemistry
University of Florida
AMBER-Developers mailing list
Received on Wed Sep 25 2019 - 09:30:03 PDT
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