tqdm (numpy.ndarray): The 2-hole-RDM as a 4-index tensor. Indices follow the internal convention of tqdm[p, q, r, s] == $$a_{p}a_{q}a_{r}^{\dagger}a_{s}^{\dagger}$$. hole_number (float): Number of holes in the system. For chemical systems this is usually the number of spin orbitals minus the number of electrons.