anchor.diff:    maxpts = 60000000
     abserr = 1e-6_dp
     relerr = 1e-3_dp
  -
  -! frequency
  -  omega = dcmplx(om,del)
   
   ! Step through 1/8 triangle of BZ
     steps=100
  
positive.code: !            Inverse of Green function at k
  !     Gkqinv real array of dimension 16 (4x4 matrix)
  !            Inverse of Green function at k+q
    common /gvars/ Pi, t, mu, x0, epsf, Gkinv, Gkqinv
    real(dp)    :: Pi, t, mu, x0, epsf
    complex(dp) :: Gkinv(16), Gkqinv(16)
  
  ! Local variables
    real(dp) :: V, Uc, Uf
    integer  :: i, j, log=10
  
  ! Set constants
    Pi=4.0_dp*datan(1.0_dp)
  
negative.code:   V   = 1e-2_dp
    Uc  = 1_dp
    Uf  = 1e-3_dp
  
  ! Set up matrix as an array of size 16
    Gkinv = (/ complex(dp) :: &
  