In connection with the current work, the convergence of the iterative solution of the CC2 response equations has been improved by determining the coefficients for the expansion of the solution vectors on the basis of the trial vectors by the requirement that the residual in the full space is minimized, instead of solving (as otherwise described in Ref. 15) the equations projected to the subspace of trial vectors. For further details, we refer to Section S6 in the SI.