of the SI depict the breaking of a single-bond in simple molecules. To assess the performance of OP-NOFT-0 in more challenging situations with many bonds or a multiple-bond, we studied the symmetric dissociation of linear H chains and the dissociation of the N$_2$ dimer. These cases have been used to test compact CI expansions controlled by seniority [@Bytautas2011]; FCI calculations are thus available for comparison.

We consider linear H chains first. These are relatively simple systems whose energy surfaces present a serious challenge for single reference methods. Fig. \[fig1\] shows the dissociation energy curve of H$_8$ obtained with different methods. OP-NOFT-0 provides a consistent description of the energy close to and everywhere above the CASSCF reference. The breakdown of CCSD(T) at large separations is caused by the single-reference character of this method. The deviation of OP-NOFT-0 from CASSCF should be attributed mainly to the restriction to the $A=0$ sector, a conclusion supported by the seniority-restricted CI calculations of Ref. [@Bytautas2011]. Close comparison with those calculations is not entirely straightforward, as Ref. [@Bytautas2011] used the slightly smaller 6-31G basis and a fixed, symmetric or broken symmetry, molecular orbital (MO) basis, whereas we used NOs that were determined self-consistently. Note that our 