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

 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:
   Organisch--Chemisches Institut       |  Zschokkestr. 12a
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