Set the self-consistent exchange-correlation functional

xc : string : Must be one of 'PZ', 'VWN', 'PW91', 'PBE', 'revPBE', 'RPBE'

Selects which density functional to use for exchange-correlation when performing electronic minimization (the electronic energy is minimized with respect to this selected functional) Notice that the electronic energy is also evaluated non-selfconsistently by DACAPO for other exchange-correlation functionals Recognized options :
  * "PZ" (Perdew Zunger LDA-parametrization) 
  * "VWN" (Vosko Wilk Nusair LDA-parametrization)
  * "PW91" (Perdew Wang 91 GGA-parametrization)
  * "PBE" (Perdew Burke Ernzerhof GGA-parametrization)
  * "revPBE" (revised PBE/1 GGA-parametrization)
  * "RPBE" (revised PBE/2 GGA-parametrization)
option "PZ" is not allowed for spin polarized calculation; use "VWN" instead.