Failure of the local exchange approximation in the evaluation of the H/sup -/ ground state

The use of the local approximation for the exchange and correlation energy within the density-functional formalism has proven to be much more successful than originally anticipated. The calculation of the ground state for weakly bound negative ions, such as H/sup -/, provides a sensitive test of the local approximation, which is known to lead to an incorrect asymptotic form of the single-particle effective potential. For H/sup -/, the result is a qualitatively incorrect ground state, the electrons being partially delocalized. We show how this problem may be corrected by constraining the electrons with a fictitious spherical barrier at a large radius. For H/sup -/ we use this procedure to obtain a ground-state energy of -0.518 a.u. and an electronic affinity for H of -0.92 eV.