In the RI-CC2 implementation in TURBOMOLE, the above response equations are effectively solved only in the space of single excitations, i.e., only the singles components of all vectors are explicitly computed and stored; contributions from the doubles are computed on-the-fly. The derivation and implementation of Eq. ( ) for (RI-)CC2 in TURBOMOLE are described in detail in the work by Fedotov et al. It boils down to iteratively solving the effective linear CC2 response equations for the singles amplitudes