anchor.diff:          use basis, only: basis_fns
           use determinants, only: det_info
           use excitations, only: excit
  -        use fciqmc_data, only: pattempt_single
  +        use fciqmc_data, only: pattempt_single, pattempt_double
           use excitations, only: find_excitation_permutation1, find_excitation_permutation2
           use hamiltonian_molecular, only: slater_condon1_mol_excit, slater_condon2_mol_excit
   
  
positive.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.
  
          ! WARNING: We assume that the excitation is actually valid.
          ! This routine does *not* calculate the correct probability that
  
negative.code:             end if
          end select
  
          pgen = 2.0_p/(nel*(nel-1)*n_aij)*(p_bija+p_aijb)
  
      end function calc_pgen_double_mol
  
  