function vc (q,a,rc,r)
c *** see RH Landau, Program lpott, 1981
c     the coulomp potential in momemtun space for moemtn t.f. q in mev,
c     r= cutoff radius,    rc=Coulomb sphere radius (all in mev-1)
c     n.b. Dont use Fermis
c     a= 3*q1*q2/(2 rc)
      Implicit Real*8 (a-h,k,m,o-z)
      b = -a/(3*rc*rc)
      If (q.gt.1.) GoTo 10
c     special q=0 form --i think this is ok
      rhl = 0.5/3.141593**2
      vc = rhl*(a*(rc**3)/3+b*(rc**5)/5+(rc*a/3)*(r*r-rc*rc))
      Return
 10   Continue
      rhl = q*rc
      s = sin(rhl)
      c = cos(rhl)
      q2 = q*q
      q3 = q2*q
      q5 = q2*q3
      rhl1 = 0.5/3.141593**2
      vc = rhl1*(a*(s-rhl*c)/q3+b*((3*q2*rc*rc-6)*s-(q3*rc**3-6*rhl)*c)/
     1q5+2*rc*a*(c-cos(q*r))/(3*q2))
      Return
      End