The spin-flip (SF) method^{497, 498, 499, 552}
is used for molecular systems with few-reference wave functions like diradicals,
bond-breaking, rotations around single bonds, and conical intersections.
Starting from a triplet (${m}_{s}=1$) ground state reference a spin-flip excitation
operator $\{{\widehat{C}}_{J}\}=\{{c}_{a\beta}^{\u2020}{c}_{i\alpha};$
${c}_{a\beta}^{\u2020}{c}_{b\sigma}^{\u2020}{c}_{i\alpha}{c}_{j\sigma},$
$\text{a < b, i j}\}$ is introduced,
which flipped the spin of one electron while singlet and
(${m}_{s}=0$) triplet excited target states are yielded. The spin-flip method
is implemented for the ADC(2) (strict and extended) and the ADC(3)
methods.^{552} Note that high-spin (${m}_{s}=1$) triplet states
can be calculated with the SF-ADC method as well using a closed-shell singlet
reference state. The number of spin-flip states that shall be calculated
is controlled with the *$rem* variable SF_STATES.