How to use the openfermion.ops.down_index function in openfermion

# Add hopping terms with neighbors to the right and bottom.
        if right_neighbor is not None:
            hubbard_model += _hopping_term(
                    up_index(site), up_index(right_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(right_neighbor), -tunneling)
        if bottom_neighbor is not None:
            hubbard_model += _hopping_term(
                    up_index(site), up_index(bottom_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(bottom_neighbor), -tunneling)

        # Add local pair Coulomb interaction terms.
        hubbard_model += _coulomb_interaction_term(
                n_spin_orbitals, up_index(site), down_index(site), coulomb,
                particle_hole_symmetry)

        # Add chemical potential and magnetic field terms.
        hubbard_model += number_operator(
                n_spin_orbitals, up_index(site),
                -chemical_potential - magnetic_field)
        hubbard_model += number_operator(
                n_spin_orbitals, down_index(site),
                -chemical_potential + magnetic_field)

    return hubbard_model

bottom_neighbor = _bottom_neighbor(
                site, x_dimension, y_dimension, periodic)

        # Avoid double-counting edges when one of the dimensions is 2
        # and the system is periodic
        if x_dimension == 2 and periodic and site % 2 == 1:
            right_neighbor = None
        if y_dimension == 2 and periodic and site >= x_dimension:
            bottom_neighbor = None

        # Add hopping terms with neighbors to the right and bottom.
        if right_neighbor is not None:
            hubbard_model += _hopping_term(
                    up_index(site), up_index(right_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(right_neighbor), -tunneling)
        if bottom_neighbor is not None:
            hubbard_model += _hopping_term(
                    up_index(site), up_index(bottom_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(bottom_neighbor), -tunneling)

        # Add local pair Coulomb interaction terms.
        hubbard_model += _coulomb_interaction_term(
                n_spin_orbitals, up_index(site), down_index(site), coulomb,
                particle_hole_symmetry)

        # Add chemical potential and magnetic field terms.
        hubbard_model += number_operator(
                n_spin_orbitals, up_index(site),
                -chemical_potential - magnetic_field)
        hubbard_model += number_operator(

singles = []
    doubles_1 = []
    doubles_2 = []

    # Get singles and doubles amplitudes associated with one
    # spatial occupied-virtual pair
    for p, q in itertools.product(range(n_virtual), range(n_occupied)):
        # Get indices of spatial orbitals
        virtual_spatial = n_occupied + p
        occupied_spatial = q
        # Get indices of spin orbitals
        virtual_up = up_index(virtual_spatial)
        virtual_down = down_index(virtual_spatial)
        occupied_up = up_index(occupied_spatial)
        occupied_down = down_index(occupied_spatial)

        # Get singles amplitude
        # Just get up amplitude, since down should be the same
        singles.append(single_amplitudes[virtual_up, occupied_up])

        # Get doubles amplitude
        doubles_1.append(double_amplitudes[virtual_up, occupied_up,
                                           virtual_down, occupied_down])

    # Get doubles amplitudes associated with two spatial occupied-virtual pairs
    for (p, q), (r, s) in itertools.combinations(
            itertools.product(range(n_virtual), range(n_occupied)), 2):
        # Get indices of spatial orbitals
        virtual_spatial_1 = n_occupied + p
        occupied_spatial_1 = q
        virtual_spatial_2 = n_occupied + r

if x_dimension == 2 and periodic and site % 2 == 1:
            right_neighbor = None
        if y_dimension == 2 and periodic and site >= x_dimension:
            bottom_neighbor = None

        # Add hopping terms with neighbors to the right and bottom.
        if right_neighbor is not None:
            hubbard_model += _hopping_term(
                    up_index(site), up_index(right_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(right_neighbor), -tunneling)
        if bottom_neighbor is not None:
            hubbard_model += _hopping_term(
                    up_index(site), up_index(bottom_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(bottom_neighbor), -tunneling)

        # Add local pair Coulomb interaction terms.
        hubbard_model += _coulomb_interaction_term(
                n_spin_orbitals, up_index(site), down_index(site), coulomb,
                particle_hole_symmetry)

        # Add chemical potential and magnetic field terms.
        hubbard_model += number_operator(
                n_spin_orbitals, up_index(site),
                -chemical_potential - magnetic_field)
        hubbard_model += number_operator(
                n_spin_orbitals, down_index(site),
                -chemical_potential + magnetic_field)

    return hubbard_model

Returns:
        mean_field_dwave_model: An instance of the FermionOperator class.
    """
    # Initialize fermion operator class.
    n_sites = x_dimension * y_dimension
    n_spin_orbitals = 2 * n_sites
    mean_field_dwave_model = FermionOperator()

    # Loop through sites and add terms.
    for site in range(n_sites):
        # Add chemical potential
        mean_field_dwave_model += number_operator(
            n_spin_orbitals, up_index(site), -chemical_potential)
        mean_field_dwave_model += number_operator(
            n_spin_orbitals, down_index(site), -chemical_potential)

        # Index coupled orbitals.
        right_neighbor = site + 1
        bottom_neighbor = site + x_dimension
        # Account for periodic boundaries.
        if periodic:
            if (x_dimension > 2) and ((site + 1) % x_dimension == 0):
                right_neighbor -= x_dimension
            if (y_dimension > 2) and (site + x_dimension + 1 > n_sites):
                bottom_neighbor -= x_dimension * y_dimension

        # Add transition to neighbor on right
        if (site + 1) % x_dimension or (periodic and x_dimension > 2):
            # Add spin-up hopping term.
            operators = ((up_index(site), 1), (up_index(right_neighbor), 0))
            hopping_term = FermionOperator(operators, -tunneling)

hubbard_model += _hopping_term(
                    up_index(site), up_index(bottom_neighbor), -tunneling)
            hubbard_model += _hopping_term(
                    down_index(site), down_index(bottom_neighbor), -tunneling)

        # Add local pair Coulomb interaction terms.
        hubbard_model += _coulomb_interaction_term(
                n_spin_orbitals, up_index(site), down_index(site), coulomb,
                particle_hole_symmetry)

        # Add chemical potential and magnetic field terms.
        hubbard_model += number_operator(
                n_spin_orbitals, up_index(site),
                -chemical_potential - magnetic_field)
        hubbard_model += number_operator(
                n_spin_orbitals, down_index(site),
                -chemical_potential + magnetic_field)

    return hubbard_model