Subject: Re: CCL:electronic correlation

From: Anatoli Korkin <[email-masked]>
Date: Fri, 4 Apr 1997 13:32:54 -0500 (EST)

> 
> Dear CCL,
> 
> My question is short: What's exactely the electronic correlation (dynamic and 
no dynamic correlation)?
> 
> 	Many thanks for your help
> 
> 				Sylvie Joanteguy
> 

Dear Silvie,

As you as a short question, I will give you a short answer :-)

The definition of the electronic correlation is simple -
it is what you get above single-determinant SCF method. Of course, you
should remeber that your result depends of the basis set you use.
Only full CI with infinite number of basis functions will give
you an exact solution for the energy and the wave fuction within 
Born-Oppenheimer approximation.

The definition of the dymanic and non-dynamic correlation is less
accurate. If you have a good SCF solution, and you do a perturbation
calculation on top of it, your correction to the energy, structure and
other properties is considered to be an effect of dynamic correlation.
If you need a few determinants to get a good first approximation for the
wave function, it is what people call non-dynamic correlation.
Of course, the destingtion between "good" and "bad" is rather uncertain,
as it happens to all qualitative concepts.

The coupled cluster (CC) method [1] provides both dynamic and non-dynamic
correlation effects, and if the number of electrons is equal to
the level of CC (e.g. CCSDT for 3 electrons) you will get the total correlation,
as the full CI does.

Anatoli Korkin

[1] see, for example, R.J. Bartlett & J.F. Stanton, Applications of
Post-Hartree-Fock Methods: A Tutorial, in Reviews in Computational
Chemistry, Vol. 5, VCH Publishers, 1994)