Subject: Re: the Pentium FDIV bug

From: "Bruce E. Wilson" <[email-masked]>
Date: Wed, 30 Nov 94 09:07:43 EST

>Based on the above, I'd suggest not panicking about the Pentium FDIV bug.
>The magnitude of the error as manifested in molecular modelling
>calculations seems almost immeasurable, and is certainly far smaller
>than the "errors" introduced by the numerous approximations that are
>used routinely (like integral cutoff thresholds or non-bonded distance
>cutoffs).
Thanks, Graham, for a reasonable evaluation of the implications of
the Pentium bug in a practical application.  The question I'd worry
about more than MD or semi-empirical methods would be ab initio
calcs.  In MD or semi-empirical, the numbers come back in rational
units, where a difference out several significant figures is well
past the practical utility of the method.  ab initio methods generate
energies in atomic units, so one is often faced with the issue that
the energy differences of interest start in the fifth or sixth
significant digit.  Even then, an error out in the 12th or 13th
digit is of comparatively small importance, but if it moves much
in from there, I'd start to be concerned.  Has anyone done the sort
of analysis on ab initio calcs with a buggy Pentium that Graham 
mentions for MD?  Tx.

                        Bruce E. Wilson ([email-masked])
                        Eastman Chemical Company