anchor.diff:          ! inefficient for small/symmetry constrained systems.
           !
           ! In:
  +        !    op_sym: symmetry of connecting operator.
           !    f: bit string representation of the Slater determinant from which
           !        an electron is excited.
           !    occ_list: integer list of occupied spin-orbitals in the determinant.
  
positive.code:         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
  
          use dSFMT_interface, only: genrand_real2
  
  
negative.code:         allowed_excitation = .false.
          do i = 1, nel
              ims = (basis_fns(occ_list(i))%Ms+3)/2
              isym = cross_product_pg_sym(basis_fns(occ_list(i))%sym, op_sym)
              if (symunocc(ims, isym) /= 0) then
                  allowed_excitation = .true.
                  exit
  