results are better than the CI results with $A=0,2$ and symmetric MO’s. OP-NOFT-0 describes the dissociation limit correctly.

OP-NOFT-0 provides not only the ground-state energy but also the 1- and 2-DM. The former displays the entanglement due to correlation through variation of the occupation numbers and the Von Neumann entanglement entropy with interatomic separation shown in Fig. S4 in Section S5 of the SI. The increase of entanglement entropy with separation signals a dramatic increase of correlation corresponding to multi-reference character. The 2-DM gives access to electron pair correlations. Fig. \[fig2\] depicts the pair-correlation function when one electron is placed outside the molecule’s right end. An asymmetric exchange-correlation hole associated with the charging of the end atom can clearly be seen. The alternating bonding/antibonding character of the links between adjacent atoms is also manifest. This reflects the instability of the open-bounded chain toward a dimerizing distortion and is evident in the pair correlations when an electron resides respectively in the mid-bond or in the mid-antibond, as shown in Fig. S5, left and right panels respectively, in Section S6 of the SI.

To test the dependence of the $\xi$ approximation [[**\[xi\_DOCI\]**]{}]{} on electron number, we studied the symmetric dissociation of H$_{16}$ 