From owner-chemistry@ccl.net Wed Jul 18 23:20:00 2012
From: "Frank Jensen frj[]chem.au.dk" <owner-chemistry[A]server.ccl.net>
To: CCL
Subject: CCL: positive energy of the frontier orbital
Message-Id: <-47236-120718195701-26904-JgQ3z0Koi+41aPndmjrwpw[A]server.ccl.net>
X-Original-From: Frank Jensen <frj]^[chem.au.dk>
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Date: Thu, 19 Jul 2012 01:56:51 +0200
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Sent to CCL by: Frank Jensen [frj#,#chem.au.dk]
A positiv HOMO energy means that the total energy can be lowered by  
removing some of the electron density. A free electron has an  
'orbital' energy of zero, thus in a sufficiently flexible basis set,  
the electron density in the HOMO can be transfered to a free electron  
state, and give a lower energy state.

This is common when using DFT for anions, where transfer of a fraction  
of the HOMO density to an unbound electron 'orbital' gives a lower  
energy state.

Note that a 'sufficiently flexible basis set' is not just your  
favorite basis set augmented with diffuse functions, it must contain  
diffuse functions with exponents in ~10^-4 or less range.

Frank




Citat af "may abdelghani may01dz[*]yahoo.fr" <owner-chemistry(!)ccl.net>:

>
>
> Dear CCL'ers
> If  the frontier orbital
> (HOMO's) energy of a compound have a positive value, is this a  
> conclusive evidence
> that this compound is a unstable
> one.
> A.May
>
>



Frank Jensen
Assoc. Prof.
Dept. of Chemistry
Aarhus University
http://old.chem.au.dk/~frj