c----------------------------------------------------------------------
      	subroutine EXCH(D,S,U,V,EX,VX)
c  GGA91 Exchange for a spin-unpolarized electronic system
c
c
c  author: Kieron Burke
c      in: D - density
c          S = ABS(GRAD D)/(2*KF*D)
c          U = (GRAD D)*GRAD(ABS(GRAD D))/(D**2 * (2*KF)**3)
c          V = (LAPLACIAN D)/(D*(2*KF)**2)
c     out: exchange energy per electron (EX) and potential (VX)
c   calls: --
      IMPLICIT REAL*8 (A-H,O-Z)
      parameter(a1=0.19645D0,a2=0.27430D0,a3=0.15084D0,a4=100.d0)
      parameter(ax=-0.7385588D0,a=7.7956D0,b1=0.004d0)
      parameter(thrd=0.333333333333D0,thrd4=1.33333333333D0)