NWXC
====

An implementation of exchange-correlation functional  for plane-wave DFT codes abstracted from the Gaussian DFT codes.
The Gaussian DFT xc codes take slightly different input and produce different gradients then what the plane-wave code does
by default.

gammaaa =  |grad nup|*|grad nup|
gammabb =  |grad ndn|*|grad ndn|
gammaab =  |grad nup|*|grad ndn|

For restricted functionals:
    Instead of outputting
       df/d|grad n|
    the DFT functionals output
       df/dgammaaa
       df/dgammabb
       

For unrestricted functionals:
   Instead of inputing
       grad n|^2 
   the gaussian dft functionals take
        gammaaa,gammabb,gammaab
       |
       
   Instead of outputting 
       df/d|grad nup|,
       df/d|grad ndn|,
       df/d|grad n|
   the DFT functionals output
       df/dgammaaa
       df/dgammabb
       df/dgammaab

