Subject: Spin contamination

From: "Michael D. Bartberger" <[email-masked]>
Date: Wed, 5 Jul 1995 11:53:35 -0400 (EDT)

Hello all:

In G92, after the value of <S^2> is printed in a UHF calculation, there 
is a value printed after this which shows <S^2> "after annihilation of 
the first spin contaminant." 

Could someone kindly explain the significance of this in terms of the 
"usefulness" of the energy or geometry obtained with the 
spin-contaminated wavefunction?

The reason for my question is this:  we are calculating some ground and 
transition states (UHF) for some (fluorinated) radical addition and 
cyclization reactions.  I find that in many of the cases the degree of spin 
contamination is too high for comfort  ( <S^2> is around 1.0 at worst. )  
However, according to G92, after annihilation of the first spin 
contaminant, this value drops to a quite acceptable 0.76.  Now, what does 
that say regarding the geometries and energies I've obtained?   I'm not 
so sure or happy about using ROHF if necessary since my understanding is 
that one can not do analytical second derivatives in G92 with ROHF..... 
with numerical second derivatives on systems of the size we are studying, 
the largest so far being C6F11(.) , I feel this would really cripple us, in 
terms of required CPU time.   The lack of account of spin polarization is 
another matter...............

I guess the main question is what is the significance of annihilation of 
that first spin contaminant?  Is the original S^2 is "bad" but the 
"annihilated" S^2 is "not so bad", then what?     :-)

I'd greatly appreciate any responses, advice, suggestions, or flames. 
(Well, go easy on the flames.)    =:^)

I'll certainly summarize if there is interest.

Best regards,

Michael
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 Michael D. Bartberger         [email-masked]        TEL: (904) 392-3580
 Department of Chemistry       [email-masked]         FAX: (904) 846-0296
 University of Florida
 Gainesville, FL  32611
 USA
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