Subject: AO coefficients / spin densities in G92

From: "Micheal Bartberger" <[email-masked]>
Date: Mon, 19 Dec 1994 21:59:06 -0500

Hello all.....

I have a question regarding the calculation of AO coefficients (eigenvectors)
for both closed-shell and radical species in G92.

My question stems from the fact that, in a semiempirical program such as MOPAC,
the eigenvectors are those obtained from a Huckel-type approximation using
the valence electrons.  For example, the 2p(z) coefficients resulting from an
AM1 calculation on ethylene have (for the HOMO) a value of .707 (the old
"square root of two over two".)  Similarly, the SOMO of the allyl radical
will have .707 at one of the CH2 termini and -.707 at the other.

My question, then, is this:  how, when using typical ab initio basis sets,
does one come up with an analagous set of AO coefficients?  Since basis sets
are comprised of more than one set of basis functions, it seems as though
each has its own "coefficient".  How does one (if at all) transform this into
some sort of "total" coefficient for a given AO at a given eigenvalue at a
given atom?   (Whew.  :)   )      I hope this isn't too off the wall......
any references, advice, would be appreciated.  The reason for asking is because
I am interested in trying to correlate the regioselectivity / stereoselectivity
of radical reactions with AO coefficients (this seems easier when using semi-
empirical programs) and with spin densities (which leads me to my next
question.......

On p. 156 and p. 175 of the Gaussian92 users manual, (which shows some sample
output of sample calculations), there is a section in the output labeled
ATOMIC SPIN DENSITIES is given.  How are these values then transformed into
a "total spin density" for each atom, which could be correlated with 
a radical's regioselectivity, and which could be transformed / compared to
hyperfine couplings in ESR.

Again, any references, pointers, scathing criticisms  ;^) are welcome.
(If it isn't blatantly obvious, I'm a little new at this.....  :)   )

I will summarize if there is sufficient interest.  Thanks again!

Regards,

Michael D. Bartberger
Department of Chemistry
University of Florida

[email-masked]
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