Re: BASIS SETS AND POLARIZATION FUNCTIONS
Certainly many (most?) chemists like to think in terms of orbitals, but
this is only a _model_ we have created -- it does not necessarily represent
reality. (I'm sure that statement will generate some replies!) And very
soon the appropriate definition of an "orbital" comes into question.
Consider that to improve the quality of an ab initio calculation, you have
two choices: improve the basis or improve the level of calculation.
If you improve the basis beyond single zeta, how do you define the orbitals?
Say I'm using a triple zeta basis. I get three times more orbitals than
the orbital picture tells me how to deal with. Sure, I can consider only
the valence orbtials, but there are properties & situations where the
core orbtials are important too. And, of course, what if you use floating
orbitals or magnetic field dependent orbitals?
Improving the level of calculation is the big killer, though. As soon
as I change from SCF to a correlated method, I only think of
"orbitals"
a place to excite electrons to or from. How does a molecule for which
correlation effects are significant (there are many examples) fit into
the orbital picture? If I tell you that the effect of triple excitations
is very important in the description of a molecule, what does that say about
the orbitals?
IMHO (that's "in my humble opinion") it safest to always think of the
basis set in ab inito calculations as mathematical functions which
are used to describe the wavefunction. Mapping results into the
orbital picture may or may not be appropriate depending on the case.
Perhaps when we have enough experience with ab initio calculations we'll
be able to develop a model which goes beyond the orbital picture by
allowing for the description of correlation effects, etc.
--
David Bernholdt bernhold ^at^ qtp.ufl.edu
Quantum Theory Project bernhold ^at^ ufpine.bitnet
University of Florida
Gainesville, FL 32611 904/392 6365
---