GENERALIZED GRADIENT APPROXIMATIONS FOR EXCHANGE AND CORRELATION - A LOOK BACKWARD AND FORWARD

Ground-state properties of many-electron systems are calculable from the exchange-correlation energy as a functional of the electron spin densities n up (r), n down (r). After a review of the local spin density (LSD) approximation, the second-order density-gradient expansion (GEA), and the generalized gradient approximation (GGA) for exchange, prospects are discussed for a simple, first-principles GGA for correlation. The new GGA for correlation, like that for exchange, is based upon a real-space cut-off of the spurious long-range part of the gradient expansion for the fluctuation hole around an electron. Scaling relations, which lead to simple functionals, are stressed.