Subject: Di'ffuse or not Di'ffuse?

From: <[email-masked]>
Date: Tue, 5 Apr 1994 16:19:17 +0600

I have a question I would like to pose.  I am interested in
calculations on the three reactions below.

(1) O + 2e ->     O(-2) EA is endothermic by 6.6 eV.
(2) SO3 + 2e -> SO3(-2) EA is endothermic by 1.4 eV.
(3) SO4 + 2e -> SO4(-2) EA is exothermic  by 2.1 eV.

My question is: Under what conditions will a basis set give
unrealistic results for a dianon?  If the HOMO is positive, does
that mean traditional SCF procedures are suspect?  Otherwise, a
lower energy solution would result if a pair of electrons were
promoted from the highest valence occupied MO to a very diffuse
orbital.

In this regard, note that Boldyrev and Simon (JPC, 1994, 98, 2298)
used 6-311+G* to describe SO4(-2).  They found that the electron in
SO4(-2) is UNBOUND with respect to SO4(-) + e but they stated that the
HOMO of SO4(-2) was negative.

Also, Guerra (CPL, 1992, 197, 205) pointed out that using diffuse
functions to describe the trimethylenemethane dianion gave an
unreliable description.  He showed that reliable results could be
obtained by leaving out the diffuse function (6-311G).

Michael McKee, Department of Chemistry, Auburn University
[email-masked]