  In this paper we propose a new pseudo-phase crystal structure, based on an
orthorhombic distortion of the diamond structure, for the ground-state
$\alpha$-phase of plutonium. Electronic-structure calculations in the
generalized-gradient approximation give approximately the same total energy for
the two structures. Interestingly, our new pseudo-phase structure is the same
as the Pu $\gamma$-phase structure except with very different b/a and c/a
ratios. We show how the contraction relative to the $\gamma$ phase, principally
in the $z$ direction, leads to an $\alpha$-like structure in the [0,1,1] plane.
This is an important link between two complex structures of plutonium and opens
new possibilities for exploring the very rich phase diagram of Pu through
theoretical calculations.
