  We study the unconventional insulating state in A_4C_{60} with a variety of
approaches, including density functional calculations and dynamical mean-field
theory. While the former predicts a metallic state, in disagreement with
experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In
that state, conduction between molecules is blocked by on-site Coulomb
repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and
important excitations (such as optical and spin gap) should be essentially
intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare
well with molecular ion values, in agreement with this picture.
