anchor.diff:                  ! 3b. Probability of generating this excitation.
   
                   ! [review] - JSS: isn't this p(a|i) p(b|ija) + p(b|i)p(a|ijb) ?
  -                ! Calculate p(ab|ij) = p(a|i) p(j|b) + p(b|i)p(a|j)
  +                ! [reply] - VAN: I think you are correct. I have changed that.
  +                ! Calculate p(ab|ij) = p(a|i) p(b|ija) + p(b|i)p(a|ijb)
                 
                   if (ij_spin==0) then 
                       ! Not possible to have chosen the reversed excitation.
  
positive.code:         ! This creates a random excitation from cdet and calculate both the probability
          ! of selecting that excitation and the Hamiltonian matrix element.
          ! Weight the double excitations according the the Cauchy-Schwarz bound
          ! <ij|ab> <= Sqrt(<ia|ai><jb|bj>)
          ! This version uses the excitation weights from the reference rather than
          ! recalculating for each determinant.
  
  
negative.code:         integer :: k
          real(p) :: weight
  
          weight_tot = 0.0_p 
          do k=1, a_list_len
              if (a_list(k) /= i) then
                  weight = sys%ueg%coulomb_int(sys%lattice%box_length(1), sys%basis, i, a_list(k))
                  weight = abs(weight)
              else
                  weight = 0.0_p
              end if
              weights(k) = weight
              weight_tot = weight_tot + weight
          end do
  