anchor.diff:  *           x_parameter: scale parameter for exchange
   *           c_parameter: scale parameter for correlation
   *
  -*   Exit - xce(*)  : PBE96 energy density
  +*   Exit - xce(*)  : revPBE energy density
   *        - fn(*,2) : d(n*xce)/dnup, d(n*xce)/dndn
   *        - fdn(*,3): d(n*xce)/d|grad nup|, d(n*xce)/d|grad ndn|
   *                    d(n*xce)/d|grad n|
  
positive.code: *           x_parameter: scale parameter for exchange
  *           c_parameter: scale parameter for correlation
  *
  *     Exit  - xce(n2ft3d) : PBE96 exchange correlation energy density
  *             fn(n2ft3d)  : d(n*xce)/dn
  *             fdn(n2ft3d) : d(n*xce/d|grad n|
  *
  
negative.code:          rss   = dsqrt(rs)
  
  *        **** calculate n*drs/dn ****
  c        rs_n = onethirdm*rs/n
           rs_n = onethirdm*rs
  
  
  
  c        **** calculate t ****
           kf = (3.0d0*pi*pi*n)**onethird
  