anchor.diff:  
               if (allowed_excitation) then
                   ! 3a. Probability of generating this excitation.
  -                pgen = calc_pgen_single_mol_no_renorm(connection%to_orb(1))
  +                pgen = pattempt_single*calc_pgen_single_mol_no_renorm(connection%to_orb(1))
   
                   ! 4a. Parity of permutation required to line up determinants.
                   call find_excitation_permutation1(cdet%f, connection)
  
positive.code: 
              if (allowed_excitation) then
                  ! 3b. Probability of generating this excitation.
                  pgen = calc_pgen_double_mol_no_renorm(connection%to_orb(1), connection%to_orb(2), ij_spin)
  
                  ! 4b. Parity of permutation required to line up determinants.
                  ! NOTE: connection%from_orb and connection%to_orb *must* be ordered.
  
negative.code:         !    determinant D_{ij}^{ab} from determinant D, assuming that (i,j)
          !    have been selected from the occupied orbitals, a is selected from
          !    an unoccupied orbital and b is selected from the set of unoccupied
          !    orbitals which conserve spin and spatial symmetry *and* given that
          !    a double excitation has been selected.
  
          ! WARNING: We assume that the excitation is actually valid.
          ! This routine does *not* calculate the correct probability that
  