“…Typically, the pCCD molecular orbitals are optimized, ,,, which re-establishes size consistency and yields localized and symmetry-broken orbitals that allow us to simulate quantum states with (quasi-)degeneracies . Numerical examples comprise bond-breaking processes in small molecules, ,,− heavy-element-containing compounds featuring lanthanide or actinide ,,− atoms, organic electronics, , and electronically excited states. ,− ,− Although these numerical studies support pCCD to be a promising alternative to capture static/nondynamic electron correlation effects, − a large fraction of the correlation energy cannot be captured by electron-pair states alone. This missing correlation energy is commonly attributed to so-called broken-pair states.…”