The spin-opposite scaling (SOS) approach originates from MP2 where it was
realized that the same spin contributions can be completely neglected,
if the opposite spin components are scaled appropriately.
In a similar way it is possible to simplify the second order ADC
equations by neglecting the same spin contributions in the ADC matrix,
while the opposite-spin contributions are scaled with appropriate
semi-empirical parameters.^{Hellweg:2008, Winter:2011, Krauter:2013}

Starting from the SOS-MP2 ground state the same scaling parameter
${c}_{T}=1.3$ is introduced into the ADC equations to scale the ${t}_{2}$
amplitudes.
This alone, however, does not result in any computational savings or
substantial improvements of the ADC(2) results.
In addition, the opposite spin components in the ph/2p2h and 2p2h/ph
coupling blocks have to be scaled using a second parameter ${c}_{c}$ to
obtain a useful SOS-ADC(2)-s model.
With this model the optimal value of the parameter ${c}_{c}$ has been found
to be 1.17 for the calculation of singlet excited states.^{Winter:2011}

To extend the SOS approximation to the ADC(2)-x method yet
another scaling parameter ${c}_{x}$ for the opposite spin components of the
off-diagonal elements in the 2p2h/2p2h block has to be introduced.
Here, the optimal values of the scaling parameters have been
determined as ${c}_{c}=1.0$ and ${c}_{x}=0.9$ keeping ${c}_{T}$ unchanged.^{Krauter:2013}

The spin-opposite scaling models can be invoked by setting METHOD to either SOSADC(2) or SOSADC(2)-x. By default, the scaling parameters are chosen as the optimal values reported above, i.e. ${c}_{T}=1.3$ and ${c}_{c}=1.17$ for ADC(2)-s and ${c}_{T}=1.3$, ${c}_{c}=1.0$, and ${c}_{x}=0.9$ for ADC(2)-x. However, it is possible to adjust any of the three parameters by setting ADC_C_T, ADC_C_C, or ADC_C_X, respectively.