AMBER Archive (2003)

Subject: AMBER: Implicit precision in sander vs architecture

From: Teletchéa Stéphane (steletch_at_biomedicale.univ-paris5.fr)
Date: Fri Oct 17 2003 - 10:42:41 CDT

I have recently seen in my dynamics simulations 'vlimit exceeded' on my
dual xeon although when using the same restart the calculation runs fine
on a power4 IBM.

I know that implicit double precision is used in fortran code, but i
have no idea of what it means when the calculations are made on 32-bits
systems (pentium/athlon) or on 64-bits systems (power4, mips, ...).

I'm a little bit afraid since i have the impression that these errors
only appear on 32-bits machines. Are roundoff more important there ?

Can someone detail the implecit precision each architecture involves for
floating point operations ?

Are 32-bits machines 2 times less precise than 64-bits machines ?

I know that after a certain point (as precised by Dr. R. Duke) dynamics
do diverge 'naturally' because of roundoff but does it happen more
rapidly on 32-bits systems as i'm suspecting it form a lower precision ?

Any hint would be very helpfull.

Stéphane Teletchéa

Writing his PhD ... 

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