.. _functionals:

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Exchange correlation functionals
==================================
The available LDA exchange-correlation functionals are: 

* LDA functionals
              #. VWN   

              #. Ceperly-Adler 

              #. Perdew-Zunger 81

              #. RPA

              #. Wigner Interpolation Functional

              #. Kohn-Sham exchange only (Xalpha with alpha =2/3)

              #. Gunnarsson-Lundquist

The first GGA functional (PW91) was implemented in 1991.  THe following GGA functionals are currently
available.

* GGA functionals
             #. Becke 1988 exchange functional (B88)

             #. Perdew-Wang 91 functional     (PW91)

             #. Perdew-Burke-Ernzerhof  96 functional  (PBE)

             #. Revised Perdew-Burke-Ernzerhof  96 functional  (PBE)

             #. RPBE 


The PBE functional is the default functional.  In the above lines, the words in the  bracket are the keywords that you can  
use to select the exchange and correlation functional of your choice. THis is specified in the file  called( CLUSTER) file. 
For example, to specify use the "BPW91" functional you will use **GGA-B88*GGA-PW91** in the first line in the CLUSTER file.

Work on implementation of Minnesota functionals is in progress.



.. Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals,
   B Hammer, L. Hansken, J. Norskov 
   Phys. Rev. B 59, 7413.7421 (1999)
   http://prb.aps.org/abstract/PRB/v59/i11/p7413_1