Subject: Re: CCL:EWALD PME problem

From: Rick Venable <rvenable!at!pollux.cber.nih.gov>
Date: Sat, 9 Aug 2003 17:57:30 -0400

On Fri, 1 Aug 2003, Michael Crowley wrote:
> I will check, but I am fairly sure that the
> fft dims do not need to be a multiple of number of processors,
> at least that was my intention.

I got curious and ran some CHARMM tests (short MD) where I varied the
number of processors and the integer grid size for the PM Ewald method
(FFTX etc.) and noted that there doesn't seem to be a requirement for
the grid sizes to be a multiple of the number of processors.  I examined
elapsed time and the average potential energy and fluctuations, and
observed no problems when FFTX wasn't a multiple.  I apologize if I
misled anyone.

However, there does seem to be a requirement to use 2**N processors only
with PME, i.e. 2, 4, 8, 16, etc., at least with CHARMM; attempts to run
with 6 processors gave the error message

Process aborting...
      ***** LEVEL -5 WARNING FROM <PME> *****
      ***** non 2**N PE count in transpose

so I would have expected a similar message for an attempt to use e.g. 30
processors.

As one might expect, my tests showed that the average potential energy
varied slightly with the grid spacing.  This suggests the grid size
should probably be kept the same within a simulation project, in order
to be using consistent force field parameters throughout.

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Rick Venable           29/500
FDA/CBER/OVRR Biophysics Lab
1401 Rockville Pike    HFM-419
Rockville, MD  20852-1448  U.S.A.
(301) 496-1905   Rick_Venable!at!nih.gov
ALT email:  rvenable!at!speakeasy.org
-------------------------------------
"Don't blame me, I voted for Kang."
                         Homer
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