LEARNINGS FROM EXCHANGE-CORRELATION POTENTIALS

Exchange-correlation potentials V-xc(r) can be obtained (within an add itive constant) from high-quality ab initio electron densities of atom s and small molecules. These potentials show qualitative features not captured by popular density functionals, and highlight unappreciated a spects of electron densities and density functionals. While V(xc)s gen erally scale with rho(1/3) except at very long or short range, the pro portionality is more similar to that of hydrogenic systems than to tha t of the homogeneous electron gas. The Fermi-Amaldi long-range approxi mation works well into the valence region. The size and location of th e large feature at the valence-core transition is system-dependent. Th is qualitative information can guide the search far improved density f unctionals. (C) 1998 Published by Elsevier Science B.V. All rights res erved.