Subject: Re: CCL:'electron density' vs 'electronic configurations'...

From: Carlos Silva Lopez <csilval^at^uvigo.es>
Date: Wed, 28 May 2003 00:49:31 +0200 (CEST)

Hello,
I have enjoyed a lot this question you have conveyed to the list. I am not
very proficient with the mathematics of ab initio and DFT methods but,
let's move towards the phylosophy of these methods, let's say towards the
"meaning" of these aproaches. I would say that the variational principle,
though it has given us the better possibility to construct theoretical
methods to describe the electronic structure, is the virus in the
development of the quantum chemistry. We now know that, just obeying the
border conditions, we will never have a lower than the exact energy. This
is great because it simplifies the wark dramatically, but we are now more
interested in the development of methods with more and more determinants
and with the widest basis set available than in looking for the "nature"
of the electronic structure. I mean, if we calculate the average weight of
a doubly excited configuration by means of thermostatistics we will
conclude that the energy gap between the ground state and the excited
state is so large at work temperatures that this weight should be
neglected. The use of multiconfigurational methods is justified just
because there are more variational parameters to optimize; then the work
out could be a lower energy (better approximated to the real), there are
uniquely mathematical reasons to do that. If you want a method based on
physical meaning consider pertubations.
Obviously this is just an opinion, and could be as far from reality as the
ab initio phylosophy is from the DFT's.

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   Carlos Silva Lspez
 Dept. Qummica Organica
  Universidade de Vigo
  Phone:0034 986812226
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