Analysis of separable potentials

Soft, fully separable ab initio pseudopotentials, introduced some years ago by Kleinman and Bylander [Phys. Rev. Lett. 48, 1425 (1982)], have proven to be very useful for large-scale electronic-structure and total-energy calculations. However, these pseudopotentials can induce unphysical results and destroy important chemical properties of the atom in the solid, if not constructed cautiously. We present here a detailed analysis of Kleinman-Bylander separable pseudopotentials. Two different techniques (a spectral investigation, and the logarithmic-derivative construction) allow a deeper understanding of their properties. It is shown how the above-mentioned problems can be avoided.