Operationally, a SF-TDDFT calculation consists of a configuration-interaction (CI) calculation within the extended manifold of determinants generated by single excitations combined with α → β spin flip. Unlike the SF-CIS method, however, the use of Kohn-Sham orbitals captures some dynamical correlation effects. Because the double excitations are limited, this CI problem does not incur the full O(N 6 ) cost of a singlesdoubles (CISD) calculation, but rather is only a few times more expensive than conventional LR-TDDFT. The number of determinants in the excitation space remains O(N occ N virt ), and does not grow in a way that increases with system size any faster than that LR-TDDFT.