Dear Pierre,

Which density functional d= o 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 hi= gh-valued professional in the field. There are sometimes artificial bound-e= lectron states, then electron is placed in very high levels...

HF eigenvalues are non-naturally positive in most cases= , because at Hartree=E2=80=93Fock level electron correlation is not treated= . CI and coupled-cluster methods solve this by using many-determinant wavef= unctions, describing electron correlation via admixing of the excitated sta= tes to the ground state studied, so results You obtained from them (especia= lly 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 artif= icial binding...

Regards,

Igors Mihailovs,

Institute= of Solid State Physics,

University of Latvia

2014-09-02 19:15 GMT+03:00 Pierre Archir= el pierre.archirel++u-psud.fr <owne= r-chemistry . ccl.net>:

Sent to CCL by: "Pierre=C2=A0 Archirel" [pierre.archirel~~u-psud.fr]
Dear colleagues,
I have a closed shell molecule (rather large: 12 first row atoms) and I wan= t 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 e= lectron is not bound and the SOMO built of the most diffuse gaussians of th= e basis.
The CCSD(T) and SAC-CI methods (which are affordable in my case) give a neg= ative electron affinity... but are these methods reliable? These methods us= e the HF orbitals, can they bind an electron if it is not bound at the HF l= evel?
Or shall I use CAS methods (very expansive in my case) which optimize orbit= als 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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