A zigzag-edged graphene nanoribbon (ZGNR) can be indexed by a width, N, which is the number of parallel trans-polyacetylene-like chains comprising the ribbon. Although ZGNRs with N = 4 or greater have been found to have an antiferromagnetic ground state and a ferromagnetic metastable state, how the ribbon makes the transition from N = 1 (namely, trans-polyacetylene-a one-dimensional, nonmagnetic semiconductor) to the wider ones is unclear. Using hybrid density functional theory, we show that N = 2 (polyacene) can be considered the narrowest ZGNR with an antiferromagnetic insulating ground state and a metastable, metallic ferromagnetic state. (C) 2009 Published by Elsevier B.V.