*From*: Ulrike Salzner <salzner:_:fen.bilkent.edu.tr>*Subject*: CCL: symmetry breaking*Date*: Fri, 9 Jun 2017 15:59:25 +0300

Hello,

I am comparing three large conjugated
pi-systems with density functional theory (global and range-separated hybrids).
The difference between them is that one contains phenyl rings, one thiophene,
and the last one selenophene. All three of molecules have UDFT instability with B3P86 with 30% HF exchange.

With open-shell singlet calculations and 50:50 HOMO-LUMO mixing, the phenyl and seleophene systems converge to open-shell biradical ground states that lie 0.02 and 0.12 eV below the closed-shell states. The thiophene system does not break the symmetry. This is odd because selenophene and thiophene system behave usually very similar.

In addition, excited calculations with UTDDFT give negative triplet excitation energies for all of them when the closed-shell wave are used.

As triplet excitation energies are probably underestimated with UTDDFT, I checked the ground state triplet energies. The triplets lie 1.24 eV (phen), 0.79 eV (thio), and 0.61 eV (seleno) above the closed-shell singlets. Again there is no reason why the thiophene system should differ from the other two.

Is there any way
other way to find the broken the symmetry ground state? Or - is the lack of a
broken symmetry solution after 50:50 HOMO-LUMO mixing sufficient to prove that
the ground state is closed-shell?

Thanks for
suggestions,

Ulrike

--

Assoc. Prof. Ulrike Salzner

Department of Chemistry

Bilkent University

06800 Bilkent, Ankara

Department of Chemistry

Bilkent University

06800 Bilkent, Ankara