Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)] Perdew, John P. ; Burke, Kieron ; Ernzerhof, Matthias Abstract Publication: Physical Review Letters Pub Date: February 1997 DOI: 10.1103/PhysRevLett.78.1396 Bibcode: 1997PhRvL..78.1396P