From: <steinbrt.rci.rutgers.edu>
Date: Tue, 24 Jul 2012 08:15:43 -0400 (EDT)

Hi all,

I have been trying to track down strange SHAKE failures in TI that have
been reported occasionally on the mailing list (Thanks Jodi, Ross and
Clara), but always had trouble reproducing them. I now found out a bit

(explanation of the problem follows, my question is below)

The problem is a desynchronization of coordinates on V0 and V1, and the
hardcoded dac/knut sync every 20 steps does not prevent them. It becomes
tricky because this problem occurs only when Amber is compiled with the
Intel Compilers but not with gcc. I could track it down to a point in
subroutine sc_pscale in softcore.F90 where we have

x(1,i) = x(1,i) + xmolnu - xmol

This is rigid translation of atom coordinates for pressure scaling, xmolnu
and xmol are the old and new box dimensions. The two numbers are quite
similar, so the subtraction involves loss of precision of about five
digits. That is not a problem per se, but this loss of precision results
in arbitrary lowest digits when the subtraction is done in intel-compiled
code.

e.g.

xmolnu 40.3614616595491
-xmol -40.3635833689616
should be
= -0.0021217094125
but is
= -0.002121709412506334 on process V0
and
= -0.002121709412520545 on process V1

Since the x coordinates are smaller than the box size, this gives roundoff
errors and results in SHAKE failures in some rare cases.

So now my question: What to do about it? Broadcasting the coordinates
after each step would be quite a performance hit for TI runs. An
alternative would be to artificially lose more digits:

tmp = float ( int((xmolnu - xmol)*1.0d6))/1.0d6

but that is hardly an elegant solution. The easiest alternative is to tell
people to avoid the Intel Compilers when doing TI, also not really a good
fix.

Any ideas on how to deal with this situation?

Kind Regards.

Thomas

Dr. Thomas Steinbrecher
formerly at the
BioMaps Institute
Rutgers University
610 Taylor Rd.
Piscataway, NJ 08854

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Received on Tue Jul 24 2012 - 05:30:02 PDT
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