anchor.diff:  
       pure function calc_pgen_single_mol_no_renorm(a) result(pgen)
   
  +        ! In:
  +        !    a: previously unoccupied orbital into which an electron is excited.
  +        ! Returns:
  +        !    The probability of generating the excitation i->a, where we select
  +        !    i uniformly from the list of occupied orbitals and a is selected
  +        !    from the list of orbitals (both occupied and unoccupied) which
  +        !    conserve spin and spatial symmetry.  Note that the probability is
  +        !    actually independent of i due to the uniform selection of i.
  +
  +        ! WARNING: We assume that the excitation is actually valid. 
  +        ! This routine does *not* calculate the correct probability that
  +        ! a forbidden excitation (e.g. due to a actually being occupied) is
  +        ! generated.  The correct way to handle those excitations is to
  +        ! explicitly reject them.  The generation probabilites of allowed
  +        ! excitations correctly take into account such rejected events.
  +
           use basis, only: basis_fns
           use fciqmc_data, only: pattempt_single
           use system, only: nel
  
positive.code:     end subroutine choose_ij_mol
  
      subroutine choose_ab_mol(f, sym, spin, symunocc, a, b, allowed_excitation)
  
          use basis, only: basis_length, basis_fns, bit_lookup, nbasis
          use system, only: nel, sym0, nsym
  
negative.code: 
      subroutine find_ia_mol(f, occ_list, i, a, allowed_excitation)
  
          ! Randomly choose a single excitation, i->a, of a determinant for
          ! molecular systems.  This routine does not reject a randomly selected
          ! a if that orbital is already occupied, which makes the excitation
          ! generation and calculation of the generation probability simpler and
          ! faster.  The downside is that the sampling is substantially more
          ! inefficient for small/symmetry constrained systems.
          !
          ! In:
          !    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.
          ! Out:
          !    i: orbital in determinant from which an electron is excited.
          !    a: previously unoccupied orbital into which an electron is excited.
          !        Not necessarily set if allowed_excitation is false.
          !    allowed_excitation: false if there are no possible single
          !        excitations from the determinant which conserve spin and spatial
          !        symmetry or if a is already occupied.
  
          use basis, only: basis_length, basis_fns, bit_lookup
          use point_group_symmetry, only: nbasis_sym_spin, sym_spin_basis_fns
          use system, only: nel
  