CCL: positive energy of the frontier orbital

[Frank Jensen <frj]^[chem.au.dk>]

 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