Subject: RE: CCL:Benchmarks and software

From: "MARTEN, BRYAN" <[email-masked]>
Date: Wed, 21 Apr 1999 14:54:00 -0400

Steven Bachrach wrote:

>(Obviously all ab intio codes should provide the same energy
>for a HF/6-31G* calculation of methane, right?)

I'm all for lots of relevant benchmarks but you might be surprised to know
that
the answer to your question is "it depends".  It depends on how many sig
figs
you want in your HF/6-31G* calculation of methane.

Few if any QM codes perform all 2e- integrals, for example.  They avoid
computing batches of them based on estimates of how large they are likely to
be
(small ones get skipped and the value of that cutoff is either hard-wired or
user-defined with some default).  Also, one program may default to
iteratively
converging an energy to 1.0E-5h while another defaults to 1.0E-10h (numbers
made
up for argument's sake).  These are but two of several possible parameters
which
can be adjusted for a balance of speed and accuracy.

For some computer experiments you need a tightly-converged ground-state
wavefunction before moving on to a higher level calculation.  For other
experiments, you clearly don't.

For people who want to compare to many decimal places, there will always be
the
additional issue of floating point round-off which will cause results to
differ
slightly from machine to machine and compiler to compiler for the same
software
program.

So even this benchmark, which appears to be well defined, will need to be
discussed to define the issues and produce benchmarks which are relevant to
people's needs.

Bryan Marten, PhD
Schering-Plough Research Institute
[email-masked]

---

From: [email-masked]
Date: Wed, 21 Apr 1999 17:10:26 +0100

MARTEN, BRYAN wrote:
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So, from a practical, [industrial], point of view the question really 
becomes a mixture of machine speed, algorithms, what one wants to
calculate, how many sig figures, how responsive the hardware and
software vendors are to problems and questions, and how clever the user
is...  One is looking for an answer to a chemical problem...

John McKelvey