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

n_spatial_orbitals = n_qubits // 2
    n_occupied = int(numpy.ceil(n_electrons / 2))
    n_virtual = n_spatial_orbitals - n_occupied

    # Unpack amplitudes
    n_single_amplitudes = n_occupied * n_virtual
    # Single amplitudes
    t1 = packed_amplitudes[:n_single_amplitudes]
    # Double amplitudes associated with one spatial occupied-virtual pair
    t2_1 = packed_amplitudes[n_single_amplitudes:2 * n_single_amplitudes]
    # Double amplitudes associated with two spatial occupied-virtual pairs
    t2_2 = packed_amplitudes[2 * n_single_amplitudes:]

    # Initialize operator
    generator = FermionOperator()

    # Generate excitations
    spin_index_functions = [up_index, down_index]
    # Generate all spin-conserving single and double excitations derived
    # from one spatial occupied-virtual pair
    for i, (p, q) in enumerate(
            itertools.product(range(n_virtual), range(n_occupied))):

        # Get indices of spatial orbitals
        virtual_spatial = n_occupied + p
        occupied_spatial = q

        for spin in range(2):
            # Get the functions which map a spatial orbital index to a
            # spin orbital index
            this_index = spin_index_functions[spin]

Args:
        n_spatial_orbitals: number of spatial orbitals (n_qubits + 1 // 2).

    Returns:
        operator (FermionOperator): corresponding to the s+ operator over
        n_spatial_orbitals.

    Note:
        The indexing convention used is that even indices correspond to
        spin-up (alpha) modes and odd indices correspond to spin-down (beta)
        modes.
    """
    if not isinstance(n_spatial_orbitals, int):
        raise TypeError("n_orbitals must be specified as an integer")

    fermion_identity = FermionOperator(())
    operator = (s_minus_operator(n_spatial_orbitals) *
                s_plus_operator(n_spatial_orbitals))
    operator += (sz_operator(n_spatial_orbitals) *
                 (sz_operator(n_spatial_orbitals) + fermion_identity))
    return operator

# Make sure we're dealing with a fermion operator from a molecule.
    if not fermion_operator.is_two_body_number_conserving():
        raise OperatorSpecificationError(
            'Operator is not two-body number conserving.')

    # Normal order and begin looping.
    normal_ordered_input = normal_ordered(fermion_operator)
    chemist_ordered_operator = FermionOperator()
    for term, coefficient in normal_ordered_input.terms.items():
        if len(term) == 2 or not len(term):
            chemist_ordered_operator += FermionOperator(term, coefficient)
        else:
            # Possibly add new one-body term.
            if term[1][0] == term[2][0]:
                new_one_body_term = (term[0], term[3])
                chemist_ordered_operator += FermionOperator(
                    new_one_body_term, coefficient)
            # Reorder two-body term.
            new_two_body_term = (term[0], term[2], term[1], term[3])
            chemist_ordered_operator += FermionOperator(
                new_two_body_term, -coefficient)
    return chemist_ordered_operator

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)
            mean_field_dwave_model += hopping_term
            mean_field_dwave_model += hermitian_conjugated(hopping_term)
            # Add spin-down hopping term
            operators = ((down_index(site), 1),
                         (down_index(right_neighbor), 0))
            hopping_term = FermionOperator(operators, -tunneling)
            mean_field_dwave_model += hopping_term
            mean_field_dwave_model += hermitian_conjugated(hopping_term)

            # Add pairing term
            operators = ((up_index(site), 1),
                         (down_index(right_neighbor), 1))
            pairing_term = FermionOperator(operators, sc_gap / 2.)
            operators = ((down_index(site), 1),
                         (up_index(right_neighbor), 1))
            pairing_term += FermionOperator(operators, -sc_gap / 2.)
            mean_field_dwave_model -= pairing_term
            mean_field_dwave_model -= hermitian_conjugated(pairing_term)

        # Add transition to neighbor below.
        if site + x_dimension + 1 <= n_sites or (periodic and y_dimension > 2):
            # Add spin-up hopping term.

def _majorana_term_to_fermion_operator(term):
    converted_term = FermionOperator(())
    for index in term:
        j, b = divmod(index, 2)
        if b:
            converted_op = FermionOperator((j, 0), -1j)
            converted_op += FermionOperator((j, 1), 1j)
        else:
            converted_op = FermionOperator((j, 0))
            converted_op += FermionOperator((j, 1))
        converted_term *= converted_op
    return converted_term

# A single round of odd-even transposition sort.
    for i in range(parity, n_qubits - 1, 2):
        # Always keep the max on the left to avoid having to normal order.
        left = max(input_ordering[i], input_ordering[i + 1])
        right = min(input_ordering[i], input_ordering[i + 1])

        # Calculate the hopping operators in the Hamiltonian.
        left_hopping_operator = FermionOperator(
            ((left, 1), (right, 0)), hamiltonian.terms.get(
                ((left, 1), (right, 0)), 0.0))
        right_hopping_operator = FermionOperator(
            ((right, 1), (left, 0)), hamiltonian.terms.get(
                ((right, 1), (left, 0)), 0.0))

        # Calculate the two-number operator l^ r^ l r in the Hamiltonian.
        two_number_operator = FermionOperator(
            ((left, 1), (right, 1), (left, 0), (right, 0)),
            hamiltonian.terms.get(
                ((left, 1), (right, 1), (left, 0), (right, 0)), 0.0))

        if not external_potential_at_end:
            # Calculate the left number operator, left^ left.
            left_number_operator = FermionOperator(
                ((left, 1), (left, 0)), hamiltonian.terms.get(
                    ((left, 1), (left, 0)), 0.0))

            # Calculate the right number operator, right^ right.
            right_number_operator = FermionOperator(
                ((right, 1), (right, 0)), hamiltonian.terms.get(
                    ((right, 1), (right, 0)), 0.0))

            # Divide single-number terms by n_qubits-1 to avoid over-accounting

for momenta_indices in grid.all_points_indices():
                momenta = grid.momentum_vector(momenta_indices)
                momenta_squared = momenta.dot(momenta)
                if momenta_squared == 0:
                    continue

                cos_index = momenta.dot(coordinate_j - coordinate_p)
                coefficient = (prefactor / momenta_squared *
                               periodic_hash_table[nuclear_term[0]] *
                               numpy.cos(cos_index))

                for spin_p in spins:
                    orbital_p = grid.orbital_id(pos_indices, spin_p)
                    operators = ((orbital_p, 1), (orbital_p, 0))
                    if operator is None:
                        operator = FermionOperator(operators, coefficient)
                    else:
                        operator += FermionOperator(operators, coefficient)
    return operator

def _diagonal_coulomb_hamiltonian_to_fermion_operator(operator):
    fermion_operator = FermionOperator()
    n_qubits = count_qubits(operator)
    fermion_operator += FermionOperator((), operator.constant)
    for p, q in itertools.product(range(n_qubits), repeat=2):
        fermion_operator += FermionOperator(
            ((p, 1), (q, 0)),
            operator.one_body[p, q])
        fermion_operator += FermionOperator(
            ((p, 1), (p, 0), (q, 1), (q, 0)),
            operator.two_body[p, q])
    return fermion_operator