  Recent high pressure work has suggested that elemental barium forms a high
pressure self-hosting structure (Ba IV) involving two `types' of barium atom.
Uniquely among reported elemental structures it cannot be described by a single
crystalline lattice, instead involving two interpenetrating incommensurate
lattices. In this letter we report pseudopotential calculations demonstrating
the stability and the potentially disordered nature of the `guest' structure.
Using band structures and nearly-free electron theory we relate the appearance
of Ba IV to an instability in the close-packed structure, demonstrate that it
has a zero energy vibrational mode, and speculate about the structure's
stability in other divalent elements.
