Effect of semicore banding on heavy-alkali-metal lattice constants : corrections to the frozen-core approximation

Equilibrium lattice constants and bulk moduli of the heavy alkali metals K, Rb, and Cs were calculated using the Troullier-Martins pseudopotentials and plane-wave basis functions. The treatment of the outermost p-shell electrons, as Bloch states, yielded lattice constants 2-3% larger than those obtained within the frozen-core approximation, including the partial core correction by Louie, Froyen, and Cohen. This narrows down a long-standing discrepancy between local-density-functional theory and experiment. Predicted bulk moduli are 30-50% larger than the measured values, within either treatment. The band dispersion of semicore states (with bandwidths 0.067, 0.14, and 0.25 eV for K, Rb, and Cs) is attributed primarily to core-electron-conduction-electron hybridization, rather than direct core-core overlap. The semicore density of states has a flat line shape, rather than the peaked shape expected for an idealized tight- binding band.