def make_Hamiltonian(pauli_list):
        """Compute the Hamiltonian.

        pauli_list is a list of tuples [(coefficient, Pauli(v,w))]
        WARNING. This is exponential in the number of qubits.
        """
        Hamiltonian = 0
        for p in pauli_list:
            Hamiltonian += p[0]*p[1].to_matrix()
        return Hamiltonian