c----------------------------------------------------------------------
      SUBROUTINE CORLSD(RS,ZET,EC,VCUP,VCDN,ECRS,ECZET,ALFC)
c  uniform-gas correlation of Perdew and Wang 1991
c
c
c  author: Perdew and Wang, 1991
c      in: rs  - seitz radius
c          zet - relative spin polarization
c     out: ec  - correlation energy per electron
c          vcup,vcdn -  up- and down-spin potentials
c          ecrs, eczet - derivatives of ec wrt rs & zet
c          alfc - correlation contribution to the spin stiffness
c   calls: GCOR
      IMPLICIT REAL*8 (A-H,O-Z)
      parameter(gam=0.5198421D0,fzz=1.709921D0)
      parameter(thrd=0.333333333333D0,thrd4=1.333333333333D0)