Quadruples natural orbitals (QNOs) are formed when D ijkl is diagonalized, and they are truncated by occupation number cutoff T CUT QNO (default 10 -6 ) in our work. The relatively looser cutoffs used for QNOs is explained by shorter range effects of coupled-cluster quadruples (much of the CI quadruples is accounted for by T 2 2 , T 3 T 1 , T 2 T 2 1 , and T 4 1 ).