Subject: CCL: a question about Koopmans' theorem

From: "Dr. Daniel Glossman-Mitnik" <dglossman~at~prodigy.net.mx>
Date: Fri, 07 Jan 2005 10:05:31 -0700

Dear netters:
 
I am a bit confused about Koopmans’ theorem. In Essentials of
Computational
Chemistry,  C.J. Cramer, Wiley, 2002, we can read on page 181:
“…Koopmans’
theorem equates the energy of the HOMO with the IP…”. On page 182, we
can read “…Koopmans´ theorem can be formally applied to electron
affinities
(EAs) as well. i.e., the EA can be taken to be the orbital energy of the
lowest
unoccupied (virtual) orbital…”
 
That is  IP = -E(HOMO)   and  EA = E(LUMO) 
 
But many articles, quote the opposite for the EA.
 
As an example, in Reactivity Descriptors, PK Chattaraj et al, in the
book
Computational Medicinal Chemistry for Drug Discovery, we can read on
page 302: “… In ab initio MO theory, I and A can be approximated as the 
negative of energies of HOMO and LUMO, respectivelu, using Koopmans’
theorem…”
 
That is  IP = -E(HOMO)  and  EA = -E(LUMO)
 
Beyond the validity of this approximation, specially for anions, which
one
is correct :    EA = E(LUMO)   or  EA = -E(LUMO)  ??
 
Thanks for your help
 
                                            Daniel 

************************************************************
          Dr. Daniel Glossman-Mitnik
             Grupo NANOCOSMO - CIMAV
   Nanociencia y Nanotecnología Computacional
          Simulación y Modelado Molecular
Miguel de Cervantes 120 - Comp. Ind. Chihuahua
        Chihuahua, Chih. 31109 - MEXICO
Tel.: (52) 614 4391151     FAX: (52) 614 4391130
Part.: (52) 614 4987113    Cel.: (52) 614 1635322
   E-mail:   <mailto:daniel.glossman~at~cimav.edu.mx>
daniel.glossman~at~cimav.edu.mx
                 <mailto:dglossman~at~prodigy.net.mx>
dglossman~at~prodigy.net.mx
*************************************************************