Results of electronic-structure calculations on the experimentally observed low-temperature monoclinic a phase of Pu are presented and compared to calculations for fcc (δ phase) and fictitious hexagonal Pu. Energy-volume curve: for fcc and hexagonal phases of Pu reveal a tendency towards more open structures, primarily due to an increased occupation of the 6d band, which has a strongly itinerant character. Bonding in these structures is not dominated by the 5 f band. However, in the experimental a phase, which is computed to be the most stable, the arrangement of the atoms in the unit cell is such that 5 f bonding is strongly enhanced when compared with fcc and hexagonal Pu