  The ground states of the actinides and their compounds continue to be matters
of considerable controversies. Experimentally, Americium-I (Am-I) is a
non-magnetic dhcp metal whereas theoretically an anti-ferromagnetic ground
state is predicted. We show that hybrid density functional theory, which
admixes a fraction of exact Hartree-Fock (HF) exchange with approximate DFT
exchange, can correctly reproduce the ground state properties of Am. In
particular, for a 0.40 fraction of HF exchange we obtain a non-magnetic ground
state with equilibrium atomic volume, bulk modulus, 5f electron population, and
the density of electronic states all in good agreement with experimental data.
We argue that the exact HF exchange corrects the overestimation of the
approximate DFT exchange interaction.
