Subject: CCL:Re:CC versus CI

From: Steven Creve <[email-masked]>
Date: Fri, 30 Oct 1998 08:23:07 +0100 (NFT)

On Thu, 29 Oct 1998, David Heisterberg wrote:

> >Suppose we perform a CI calculation with all determinants that are
> >normally generated by CCSD (exp(T1+T2)). Would the obtained result be
> >worse or better than the CCSD value?
> 
> Doesn't exp(T1) in fact generate all possible excitations?  The CI
> case would thus be full CI and therefore exact.  I think the coupled
> cluster wave operator, as long as it includes singles, must generate
> all possible excitations, but most of them have their coefficients
> determined as functions of the coefficients of lower order excitations.
> 
> Dave Heisterberg
> 

Truncated CCSD does *not* generate all excitations. By making use of the
hausdorff comutator expansion, one can show that the exp-series terminates
exactly after a certain number of orders.

steven

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Steven Creve                       [email-masked]
Labo Quantumchemie
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