Different versions of the Generalized Gradient Approximation (GGA) are discussed in the following articles:

**PW91**:**Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation**

J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson, D. J. Singh, and C. Fiolhais

Phys. Rev. B 46, 6671 (1992) (subscription needed).

PW91 has as an ingredient the LDA correlation and a choice need to be made between VWN, PZ, and PW.**PBE**:**Generalized Gradient Approximation Made Simple**

J. P. Perdew, K. Burke, and M. Ernzerhof

Phys. Rev. Lett. 77, 3865 (1996) (subscription needed).

PBE has as an ingredient the LDA correlation and a choice need to be made between VWN, PZ, and PW.**BLYP**:

Exchange:**Density-functional exchange-energy approximation with correct asymptotic behavior**

A. D. Becke

Phys. Rev. A 38, 3098 (1988) (subscription needed).

Correlation:**Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density**

C. Lee, W. Yang, and R. G. Parr

Phys. Rev. B 37, 785 (1988) (subscription needed).