Stability of atomic and diatomic hydrogen in fcc palladium

First-principles self-consistent total energy calculations for various configurations of atomic and diatomic hydrogen inside fcc palladium have been carried out to clarify some of the solid state aspects of the physics of dense hydrogen in metals. We find that the stability of the system is controlled by the relative position of the hydrogen-induced antibonding level with respect to the Fermi energy. Diatomic H ₂ in crystalline palladium is shown to have but metastable local minima whose internuclear separation are yet larger than that of the isolated H ₂ molecule.