Show that the onebody part of the Hamiltonian $$ \hat{H}_0 = \sum_{pq} \langle p|\hat{h}_0|q\rangle a^{\dagger}_p a_q, $$ can be written, using standard annihilation and creation operators, in normal-ordered form as $$ \hat{H}_0 = \sum_{pq} \langle p|\hat{h}_0|q\rangle \left\{a^\dagger_p a_q\right\} + \sum_i \langle i|\hat{h}_0|i\rangle. $$ Explain the meaning of the various symbols. Which reference vacuum has been used?