Local-density theory of mixed-valence TmSe and the valence transition in Tm chalcogenides

The first precise ab initio total-energy local-density calculation of the electronic structure of a mixed-valence compound, TmSe, correctly reproduces the observed lattice constant and a nonintegral f number for the ground state. The mixed-valence--to--integral-valence transition in the chalcogenides is shown to be connected to the opening of a gap between the f band and the conduction d band. Experimental volume-pressure relations cannot be explained without taking into account the interaction with the valence p band.