  We examine the electronic ground state of acenes with different number of
fused benzene rings (up to 40) by using first principles density functional
theory. Their properties are compared with those of infinite polyacene. We find
that the ground state of acenes that consist of more than seven fused benzene
rings is an antiferromagnetic (in other words, open-shell singlet) state, and
we show that this singlet is not necessarily a diradical, because the spatially
separated magnetizations for the spin-up and spin-down electrons increase with
the size of the acene. For example, our results indicate that there are about
four spin-up electrons localized at one zigzag edge of 20-acene. The reason
that both acenes and polyacene have the antiferromagnetic ground state is due
to the zigzag-shaped boundaries, which cause pi-electrons to localize and form
spin orders at the edges. Both wider graphene ribbons and large
rectangular-shaped polycyclic aromatic hydrocarbons have been shown to share
this antiferromagnetic ground state. Therefore, we demonstrate that the
pi-electronic structure of higher acenes and ployacene are still dictated by
the zigzag edges, and our results provide a consistent description of their
electronic ground state.
