“…40 The resulting orbital optimized (oo)-pCCD molecular basis is not restricted by the active space size as for standard multireference methods and, therefore, provides an efficient treatment of electron correlation effects. Numerical examples include bond-breaking processes in small molecules, 28,41–48 compounds containing heavy elements like lanthanides 49 and actinides, 42,47,50–55 organic electronics, 56,57 electronically excited 51–53,58–60 and ionized states. 61–63…”