Hamiltonian

Thus, the Hamiltonian can be written in terms of Pauli matrices as $$ \begin{align*} H = \sum_p\left(2\delta_p+g_{pq}\right)\left(\frac{I_p-Z_p}{2}\right) +\sum_{p < q}g_{pq}\frac{X_pX_q+Y_pY_q}{2} \end{align*} $$