  We have investigated the interplay between magnetic and structural degrees of
freedom in elemental Mn. The equilibrium volume is shown to depend critically
on the magnetic interactions between the Mn atoms. While the standard
generalized-gradient-approximation underestimates the equilibrium volume, a
more accurate treatment of the effects of electronic localization and magnetism
is found to solve this longstanding problem. We capture well the complexity of
the large 58 atoms per unit cell $\alpha$-Mn system for the first time,
including its charge and spin patterns and the canting of spins with respect to
the average magnetization direction.
