which quantifies the amount of entanglement between a specific spin-orbital i and the other orbitals (Figure ). Here, the one-orbital reduced density matrix ρ i is a four-dimensional density matrix with H i " spant | y, | Òy, | Óy, | ÒÓy u, that can be obtained by tracing out all other orbital contributions from the many particles reduced density matrix except those of orbital i. The diagonal matrix elements of the one-orbital RDM for (spatial) orbital i can be calculated from the expectation values of generalized correlation functions which can also be expressed via proper combinations of the expectation value of standard occupation number operators .