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GGA functionals
GGA functionals provide a better overall description of the electronic subsystem than the LDA functionals. The LDA description tends to overbind atoms, so that the bond lengths and the cell volume are usually underestimated by a few percent and the bulk modulus is correspondingly overestimated. GGA corrects this error but may underbind instead, leading to slightly long bond lengths.

PBE (Perdew et al., 1996) is the default exchange-correlation functional. It is recommended, especially, for studies of molecules interacting with metal surfaces, although it is also fairly reliable for bulk calculations. The PW91 (Perdew et al., 1992) functional should be used for comparisons with literature data, as it is the most widely used GGA functional. The results obtained with the PW91 functional should be very similar to those obtained with PBE. Another nonlocal functional, RPBE (Hammer et al., 1999), is a revised form of the PBE functional designed to improve the description of metallic surfaces. The latest GGA potential, WC, is due to Wu and Cohen (2006). The authors claim significant improvements for lattice constants, crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA.