Subject: CCL: Orbitals

From: assfeld-.at.-host20.lctn.uhp-nancy.fr
Date: Fri, 30 May 2003 14:06:18 +0200

Hey,

Just to throw oil on fire....

Of course orbitals (molecular, atomic...) are not "real", because they
just result from the mathematical construction of our theory. They are
not unique, and so on...
But is the many electrons wave function (Y) more "real"?
   Parentheses:
Ok, I know that wave function are not real, only the square (YY*) has a
physical meaning. But I prefer to speak about wave function, knowing
that 
the observable density is deduced from this wave function.
   End Parentheses.
What I mean is that:
    - The exact Hamiltonien (fully relativistic, not B-O,...) is
generally 
not used (not known?).
Then the exact solutions we have are just solution of an approximate
Hamiltonien, and thus can not describe exactly the physical phenomenon.
    - The exact solutions of the (even approximate) Hamiltonien are not
known.
What we get, is just a more or less acceptable representation of
"reality".
Does that mean that everything we get with Hartree-Fock method is
garbage?
The answer is no, because a lot of the "physic" of the system is
included 
in the theory, but it could be improved...

Does someone remember (or know) why we use orbitals in the first time?
I think that it is because they can be exact solutions of approximate
Hamiltonien! If, in a model representation of the world surrounding us, 
we consider that all electrons of a system can move more or less 
independently from one another, then orbitals are "real" in the sense 
that they allow us to describe the world with a given level of accuracy.
Of course it is not perfect. 
I really do think that canonical orbitals are "real" because they come 
> from a theory that has the major physical factor in it (symmetry,
nuclear
attraction, kinetic energy, average coulombic interaction...). Thus, if
one improve the one-electron Hamiltonien (for example  with Kohn-Sham
method) that gives the canonical orbitals, then orbitals will be more 
"real" in the sense that they will predict or explain more accurately 
the world...

Well, that was just my 2 eurocents.

PS: In reply to E. Lewars, I will even say that virtual orbitals are
"real"
 because they allow us to explain spectroscopy (n -> Pi* transition in
H2CO
 for example)

                                      ...Xav

Pr. Xavier Assfeld             Xavier.Assfeld-.at.-lctn.uhp-nancy.fr
Laboratoire de Chimie thiorique     (T) 33 3 83 68 43 74
Universiti Henri Poincari           (F) 33 3 83 68 43 71
F-54506 Nancy B.P. 239              http://www.lctn.uhp-nancy.fr