Subject: RE: How good are non variational methods ?

From: "Shobe, Dave" <[email-masked]>
Date: Fri, 22 Nov 2002 09:27:31 -0500

The example of CCSD(T) shows that nonvariational methods can have high
accuracy.

It seems to me that in the past few decades, quantum chemical methods
developers have valued size consistency over variational-ness (I forget what
the actual word is).  After all if the method is not size-consistent, how
can we trust it for dimerizations, dissociation energies, or any other
reaction where groups of atoms come together or apart?  And apparently it's
difficult to find a level of theory other than HF and Full CI which is both
size-consistent and variational.

--David Shobe
Süd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  [email-masked]

Don't bother flaming me: I'm behind a firewall.

-----Original Message-----
From: Patrick Bultinck [mailto:[email-masked]]
Sent: Friday, November 22, 2002 8:12 AM
To: [email-masked]
Subject: CCL:How good are non variational methods ?

Dear all,

I am curious in your ideas about what good non variational methods are in
quantum chemistry ? In variational techniques (e.g. Hartree-Fock, CI) on
minimizes the variational integral and through the variation theorem you
know you will almost stay just above or equal to the true solution (at
least for the energy). With e.g. perturbation theory, you may end up
anywhere, that is: above, below or on the dot of the true solution.

What is your opinion on techniques such as Coupled Cluster (iterative but
in most formulations not variational, although I naively think this must be
possible), or MP2, MP4, ... ? We often see "highly accurate" results of say
CCSD(T) (using perturbation theory on top of the iterative scheme), but how
to put this in perspective knowing that there is no variational theorem
applicable ?

Has this ever been discussed throughly in some reference ? What are the
experiences of the CCL members ?

Thanks for your ideas/time,

Patrick Bultinck
Quantum Chemistry Group
Ghent University
Belgium

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