Subject: RE: Summary: DFT and Koopmans' theorem

From: "Shobe, Dave" <[email-masked]>
Date: Mon, 19 Feb 2001 18:11:21 -0500

Perhaps Koopman's theorem energies are like Mulliken charges--they aren't
accurate in themselves, but they do tend to correlate with more "physical"
measures of ionizability (or nucleophilicity/electrophilicity) and atomic
charge respectively.  
Is this a reasonable statement?  [Hope I'm not starting a flame war about
the different ways of defining atomic charges...]
--David Shobe 
Süd-Chemie Inc. 
phone (502) 634-7409 
fax     (502) 634-7724 
email  [email-masked] <mailto:[email-masked]>  

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]
Sent: Monday, February 19, 2001 12:15 PM
To: [email-masked]
Subject: CCL:Summary: DFT and Koopmans' theorem

--- You wrote: 

so what? Koopman's theorem is no theorem and does not hold for anything, at
least not in a mathematical sense. For Hartree-Fock, it is sometimes a good
approximation because of error compensation. it is no "theorem". 

--- end of quote --- 

This attitude is reasonable if one is wondering how best to calculate IP and
EA. Koopmans does not give accurate IP and EA, a flaw that is built in to
the "theorem". 

Complete dismissal of Koopmans' theorem does not make sense to me, however,
given the large number of chemists who rely on it as a guide to chemical
reactivity. Chemists who think about chemical reactivity in terms of MO
energies (and Frontier MO properties) are relying on the notion that these
energies indicate how easily a molecule can gain/lose an electron. If you
like, you can tell them these energies indicate no such thing (an
exaggeration, yes?), but what tools would you give them to use instead?
Should the way they think about reactivity be limited to methods that give
state-of-the-art numerical accuracy? 

-Alan 

==== 

Alan Shusterman 

Department of Chemistry 

3203 SE Woodstock Blvd 

Reed College 

Portland, OR 97202 

503-771-1112, ext. 7699 

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From: Joerg Grunenberg <[email-masked]>
Date: Tue, 20 Feb 2001 16:47:00 +0100

I think that is the point.

There may be a correlation, but that does not mean automatically that there is a
causal link between them.

A correlation between experimental and theoretical vertical electron detachments
(SCIENCE,291, No.5505, p. 859) is nice, but it is no proof of the existence of
molecular orbitals, which can then be used to show aromaticity...

Jörg

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Dr. Jörg Grunenberg, Org. Chemie, Hagenring 30, 38106 Braunschweig
email: [email-masked] phone: +49 531 391 5252
URL: http://www.tu-bs.de/institute/org-chem/grunenberg/grunenberg.html

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From: "Alexandre Hocquet" <[email-masked]>
Date: Tue, 20 Feb 2001 23:36:26 +0100

Perhaps it would be interesting to know when and how Koopmans theorem has
been thus baptized, instead of something like "Koopmans approximation" ?

Aparently, the original paper is : Koopmans, T. Uber die Zuordnung von
Wellenfunktionen and Eigenwerten zu den Einzelnen Elektronen eines Atoms.
Physica, 1, 104 (1933)

Any historical insight, anyone ?

Alexandre Hocquet
Laboratoire de Physicochimie Biomoléculaire et Cellulaire
UPRESA CNRS 7033
Case courrier 138, 4 Place Jussieu, F-75252 Paris Cedex 05, France.
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