CCL:G: about electron affinity

You will always get negative electron binding energy at the HF level. Negative ions require correlation. Once you co to a post-HF method, if your molecule can bind an electron, you should be able to obtain a positive electron affinity.Â

Paulo Acioli, Chair Earth Science and PhysicsÂ

Associate Professor of PhysicsÂ

Department of Physics and Astronomy

Northeastern Illinois University

5500 North St. Louis Avenue, Chicago, IL 60625

Phone: (773) 442-4733


On Tue, Sep 2, 2014 at 11:15 AM, Pierre Archirel <owner-chemistry*> wrote:

Sent to CCL by: "Pierre Archirel" []
Dear colleagues,
I have a closed shell molecule (rather large: 12 first row atoms) and I want to know if this molecule can bind an electron.
DFT tells me "yes" with positive values of the electron affinity, in the range 0.5-1.0 eV according to the functional. I suspect that these values are false, or at least largely overestimated.
I now turn to ab initio methods, but I first see that at the HF level the electron is not bound and the SOMO built of the most diffuse gaussians of the basis.
The CCSD(T) and SAC-CI methods (which are affordable in my case) give a negative electron affinity... but are these methods reliable? These methods use the HF orbitals, can they bind an electron if it is not bound at the HF level?
Or shall I use CAS methods (very expansive in my case) which optimize orbitals at the CI level? Is it obligatory in this case?
Many thanks in advance,
Pierre Archirel
LCP, Universite Paris-Sud, Orsay, France

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