From owner-chemistry@ccl.net Sat Jun 10 19:34:01 2017
From: "Susi Lehtola susi.lehtola..alumni.helsinki.fi" <owner-chemistry : server.ccl.net>
To: CCL
Subject: CCL: symmetry breaking
Message-Id: <-52835-170610054832-28695-eaCd3yCVC/oj2NT8keVtZw : server.ccl.net>
X-Original-From: Susi Lehtola <susi.lehtola a alumni.helsinki.fi>
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Date: Sat, 10 Jun 2017 02:48:22 -0700
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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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