  Alkali doped aromatic compounds have shown evidence of metallic and
superconducting phases whose precise nature is still mysterious. In potassium
and rubidium doped phenanthrene, superconducting temperatures around 5 K have
been detected, but such basic elements as the stoichiometry, crystal structure,
and electronic bands are still speculative. We seek to predict the crystal
structure of M3-phenanthrene (M = K, Rb) using ab-initio evolutionary
simulation in conjunction with density functional theory (DFT), and find metal
but also insulator phases with distinct structures. The original P21
herringbone structure of the pristine molecular crystal is generally abandoned
in favor of different packing and chemical motifs. The metallic phases are
frankly ionic with three electrons acquired by each molecule. In the
nonmagnetic insulating phases the alkalis coalesce reducing the donated charge
from three to two per phenanthrene molecule. A similar search for K3-picene
yields an old and a new structure, with unlike potassium positions and
different electronic bands, but both metallic retaining the face-to-edge
herringbone structure and the P21 symmetry of pristine picene. Both the new
K3-picene and the best metallic M3-phenanthrene are further found to undergo a
spontaneous transition from metal to antiferromagnetic insulator when spin
polarization is allowed, a transition which is not necessarily real, but which
underlines the necessity to include correlations beyond DFT. Features of the
metallic phases that may be relevant to phonon-driven superconductivity are
underlined.
