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.. Horn, F. Plasser, T. Müller, F. Libisch, J. Burgdörfer, H. Lischka:
"A comparison of singlet and triplet states for one- and two-dimensional graphene nanoribbons using multireference theory";
Theoretical Chemistry Accounts, 133 (2014), 1511 - 1519.

This study examines the radical nature and spin
symmetry of the ground state of the quasi-linear acene and
two-dimensional periacene series. For this purpose, high-
level ab initio calculations have been performed using the
multireference averaged quadratic coupled cluster theory
and the COLUMBUS program package. A reference space
consisting of restricted and complete active spaces is taken
for the
p
-conjugated space, correlating 16 electrons with 16
orbitals with the most pronounced open-shell character for
the acenes and a complete active-space reference approach
with eight electrons in eight orbitals for the periacenes.
This reference space is used to construct the total config-
uration space by means of single and double excitations.
By comparison with more extended calculations, it is
shown that a focus on the
p
space with a 6-31G basis set is
sufficient to describe the major features of the electronic
character of these compounds. The present findings suggest
that the ground state is a singlet for the smaller members of
these series, but that for the larger ones, singlet and triplet
states are quasi-degenerate. Both the acenes and periacenes
exhibit significant polyradical character beyond the tradi-
tional diradical.

http://dx.doi.org/10.1007/s00214-014-1511-8