Hamiltonian

We consider the Hamiltonian

$$ \begin{equation} \tag{2} H = \sum_{pq} \varepsilon^p_q a^\dagger_p a_q + \frac{1}{4}\sum_{pqrs}\langle pq\vert V\vert rs\rangle a^\dagger_pa^\dagger_q a_sa_r \end{equation} $$