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
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
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?
Assoc. Prof. Ulrike Salzner
06800 Bilkent, Ankara