# CCL: symmetry breaking

*From*: Susi Lehtola <susi.lehtola a alumni.helsinki.fi>
*Subject*: CCL: symmetry breaking
*Date*: Sat, 10 Jun 2017 02:48:22 -0700

Sent to CCL by: Susi Lehtola [susi.lehtola(a)alumni.helsinki.fi]
On 06/09/2017 05:59 AM, Ulrike Salzner salzner*_*fen.bilkent.edu.tr wrote:

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?
`

`HOMO-LUMO mixing stability is a necessary but not sufficient condition
``for wave function stability. You should run full stability analysis on
``the closed-shell singlet state to find out if the solution you've found
``really is a local minimum.
` --
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Mr. Susi Lehtola, PhD Chemist Postdoctoral Fellow
susi.lehtola(a)alumni.helsinki.fi Lawrence Berkeley National Laboratory
http://www.helsinki.fi/~jzlehtol USA
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