CCL:G: about electron affinity



Dear Pierre,

Which density functional do You use? Some of them might give quite reasonable results, whereas other might not, especially if your molecule contains long conjugated chain. 1 eV does not seem to be too few for me, but in no way I could say that I am high-valued professional in the field. There are sometimes artificial bound-electron states, then electron is placed in very high levels...

HF eigenvalues are non-naturally positive in most cases, because at HartreeâFock level electron correlation is not treated. CI and coupled-cluster methods solve this by using many-determinant wavefunctions, describing electron correlation via admixing of the excitated states to the ground state studied, so results You obtained from them (especially from CCSD(T) ) seem to be OK in most cases (if You have used basis set that is sufficiently large). Is the bound electron on some very high level also in CCSD(T) and CI case? If yes, this may indeed be the effect of artificial binding...

Regards,
Igors Mihailovs,
Institute of Solid State Physics,
University of Latvia



2014-09-02 19:15 GMT+03:00 Pierre Archirel pierre.archirel++u-psud.fr <owner-chemistry . ccl.net>:

Sent to CCL by: "Pierre Archirel" [pierre.archirel~~u-psud.fr]
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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