phdm (numpy.ndarray): The 2-particle-hole-RDM as a 4-index tensor. Indices follow the internal convention of phdm[p, q, r, s] == $$a_{p}^{\dagger}a_{q}a_{r}^{\dagger}a_{s}$$.
num_particles number of particles in the system.
num_basis_functions number of spin-orbitals (usually the number of qubits)