abstract
A variety of planar pi-conjugated hydrocarbons such as heptauthrene, Clar's goblet and, recently synthesized, triangulene have two electrons occupying two degenerate molecular orbitals. The resulting spin of the interacting ground state is often correctly anticipated as S = 1, extending the application of Hund's rules to these systems, but this is not correct in some instances. Here we provide a set of rules to correctly predict the existence of zero mode states as well as the spin multiplicity of both the ground state and the low-lying excited states, together with their open- or closed-shell nature. This is accomplished using a combination of analytical arguments and configuration interaction calculations with a Hubbard model, both backed by quantum chemistry methods with a larger Gaussian basis set. Our results go beyond the well established Lieb's theorem and Ovchinnikov's rule, as we address the multiplicity and the open-/closed-shell nature of both ground and excited states.
keywords
GRAPHENE NANOFLAKES; GROUND-STATE; CLAR-GOBLET; KEKULE; CHARACTER
subject category
Chemistry; Science & Technology - Other Topics; Materials Science; Physics
authors
Ortiz, R; Boto, RA; Garcia-Martinez, N; Sancho-Garcia, JC; Melle-Franco, M; Fernandez-Rossier, J
our authors
Manuel Melle-Franco
Principal Researcher
Roberto Álvarez Boto
Post-Doc Fellowshipacknowledgements
We acknowledge J. L. Lado for fruitful discussions. J.F.-R and R.O. acknowledge financial support from MINECO-Spain (grant no. MAT2016-78625-C2) and from the Portuguese Fundacao para a Ciencia e a Tecnologia (FCT) for the project P2020-PTDC/FIS-NAN/4662/2014. J.F.-R., M.M.-F. and N.G.-M. acknowledge support from the P2020-PTDC/FIS-NAN/3668/2014. J.F.-R. acknowledges support from UTA-PEXPL/NTec/0046/2017 projects as well as Generalitat Valenciana funding (Prometeo2017/139). R.O. and J.C.S.-G. acknowledge ACIF/2018/175 (Generalitat Valenciana and Fondo Social Europeo). M.M.-F. and R.B. acknowledge the Portuguese Fundacio para a Ciencia e a Tecnologia (FCT) for the project IF/00894/2015 and FCT ref. UID/CTM/50011/2019 for CICECO - Aveiro Institute of Materials. This project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no.