Subject: Re: CCL:Off-diagonal elements of the CI matrix

From: "Michael E. Beck" <[email-masked]>
Date: Thu, 6 Aug 1998 08:13:19 +0200 (MEST)

On Wed, 5 Aug 1998, Gustavo Moura wrote:

> Dear CCL readers,
> 	I am writing a program to do CI calculations on some conjugated
> molecules using semiempirical (PPP) parameters. Unfortunately, I am having
> problems to calculate the off-diagonal elements of the CI matrix. In their book
> Szabo e Ostlund show (tables 2.3 and 2.4) how to calculate these elements when
> the determinants are in maximum coincidence. I am looking for references where
> I
  Dear Sir,
  
maybe this not quite the answer you are expecting, but from your letter i
suspect that your program is in a very early stage of development. 

A very elegant way to set up the Hamiltonian in a basis of spin adapted
configurations (CCFs) is to use the graphical approach pioneered by Paldus
and Shavitt, GUGA. This approach also gives a very smooth way to construct
the configuration space. 

GUGA seems to be applied mainly in ab initio theory, but personally i
think it's very well suited for semiempirics, too. Once you have got used
to the graphs, you'll love them, promise.

Coming back to your question: In GUGA problems like "maximal coincidence" 
just do not arise. 

sincerly

Michael E. Beck

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  Dr. Michael E. Beck                  |  privat:
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