From:	Joerg Hill <jxh -AatT- msi.com>
Date:	Mon, 27 Jan 1997 11:18:41 -0800 (PST)
Subject:	Re: CCL:basis set balance?

> The question:  Can people please explain the concept of having a balanced
> basis set and the dangers inherent in an unbalanced set?  In particular, why
> can I not simply use a large basis, including extra split valance functions,
> polarization and diffuse functions, only on the atoms I need them on, and a
> smaller, more compact basis set on the "unimportant" atoms to my chemical
> question?  This would certainly save time and resources during the
calculation.

One of the dangers of using an unbalanced basis set is that you might
get electron
density where it shouldn't be. If you put a good basis only where you
think important
things will happen and a poor basis everywhere else than the electrons
which are
at the atoms with the poor basis will (partially) move to the atoms with
the good basis
(since there is more flexibility in the basis and the total energy of
the system will be
lowered). So you might get the effect you were interested in, but only
because you
described the system the way you thought it would be.

The basis set is only an approximation used to describe the electron density
distribution (since there is currently no way to solve the Schroedinger
equation
for a larger system without this approximation). So you have to use the same
quality of approximation for the whole molecule if you want to get reasonable
results.

Joerg-R. Hill

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Dr. Joerg-Ruediger Hill   | Every attempt to employ mathematical methods in the
Molecular Simulations     | study of chemical questions must be considered pro-
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E-mail jxh(-(at)-)msi.com        |     Auguste Comte, Philosophie Positive,
Paris, 1838
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