c----------------------------------------------------------------------
      	subroutine CORPBE2(RS,ZET,T,UU,VV,WW,lgga,lpot,ec,vcup,vcdn,
     1                  H,DVCUP,DVCDN)
c  Official PBE correlation code. 
c
c
c  author: K. Burke, May 14, 1996.
C  INPUT: RS=SEITZ RADIUS=(3/4pi rho)^(1/3)
C       : ZET=RELATIVE SPIN POLARIZATION = (rhoup-rhodn)/rho
C       : t=ABS(GRAD rho)/(rho*2.*KS*G)  -- only needed for PBE
C       : UU=(GRAD rho)*GRAD(ABS(GRAD rho))/(rho**2 * (2*KS*G)**3)
C       : VV=(LAPLACIAN rho)/(rho * (2*KS*G)**2)
C       : WW=(GRAD rho)*(GRAD ZET)/(rho * (2*KS*G)**2
c       :  UU,VV,WW, only needed for PBE potential
c       : lgga=flag to do gga (0=>LSD only)
c       : lpot=flag to do potential (0=>energy only)
c output: ec=lsd correlation energy from [a]
c       : vcup=lsd up correlation potential
c       : vcdn=lsd dn correlation potential
c       : h=NONLOCAL PART OF CORRELATION ENERGY PER ELECTRON
c       : dvcup=nonlocal correction to vcup
c       : dvcdn=nonlocal correction to vcdn
c  calls: --
c----------------------------------------------------------------------
c----------------------------------------------------------------------
c References:
c [a] J.P.~Perdew, K.~Burke, and M.~Ernzerhof, 
c     {\sl Generalized gradient approximation made simple}, sub.
c     to Phys. Rev.Lett. May 1996.
c [b] J. P. Perdew, K. Burke, and Y. Wang, {\sl Real-space cutoff
c     construction of a generalized gradient approximation:  The PW91
c     density functional}, submitted to Phys. Rev. B, Feb. 1996.
c [c] J. P. Perdew and Y. Wang, Phys. Rev. B {\bf 45}, 13244 (1992).
c----------------------------------------------------------------------
c----------------------------------------------------------------------
c thrd*=various multiples of 1/3
c numbers for use in LSD energy spin-interpolation formula, [c](9).
c      GAM= 2^(4/3)-2
c      FZZ=f''(0)= 8/(9*GAM)
c numbers for construction of PBE
c      gamma=(1-log(2))/pi^2
c      bet=coefficient in gradient expansion for correlation, [a](4).
c      eta=small number to stop d phi/ dzeta from blowing up at 
c          |zeta|=1.
      IMPLICIT REAL*8 (A-H,O-Z)
      parameter(thrd=1.d0/3.d0,thrdm=-thrd,thrd2=2.d0*thrd)
      parameter(sixthm=thrdm/2.d0)
      parameter(thrd4=4.d0*thrd)
      parameter(GAM=0.5198420997897463295344212145565d0)
      parameter(fzz=8.d0/(9.d0*GAM))
      parameter(gamma=0.03109069086965489503494086371273d0)
      parameter(bet=0.06672455060314922d0,delt=bet/gamma)
      parameter(eta=1.d-12)