The electronic structure of thorium is computed to study the recently reported [Phys. Rev. Lett. 67, 3563 (1991)] high-pressure fcc-bct structural transition. We find that elemental thorium is unique in that unoccupied f levels become populated at ultrahigh pressures, with the Fermi level intercepting the f band at a volume fraction of 0.6. The total energies show that the resulting occupation of f bands by about 1 electron drives the fcc-to-bct transition near 80-GPa pressure, and thorium then resembles the f-band metal cerium. The 6p electrons must be treated as band states in estimating the transition pressure and 0-K isotherm.

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