Large computational savings are possible if the virtual space is truncated using the frozen natural orbital (FNO) approach (see Section 6.11). Extension of the FNO approach to ionized states within EOM-CC formalism was recently introduced and benchmarked.518 In addition to ground-state coupled-cluster calculations, FNOs can also be used in EOM-IP-CCSD, EOM-IP-CCSD(dT/fT) and EOM-IP-CC(2,3). In IP-CC the FNOs are computed for the reference (neutral) state and then are used to describe several target (ionized) states of interest. Different truncation scheme are described in Section 6.11.