GENERALIZED GRADIENT APPROXIMATION MADE SIMPLE

Generalized gradient approximations (GGA's) for the exchange-correlati on energy improve upon the local spin density (LSD) description of ato ms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underly ing the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW9 1 include an accurate description of the linear response of the unifor m electron gas, correct behavior under uniform scaling, and a smoother potential.