A corrected CIS(D∞) method for valence and Rydberg excitation energies

CIS(D-infinity) method provides reasonable valence excitation energies, but fails for Rydberg states. A corrected CIS(D-infinity) method is proposed where excitation energies are calculated based on the energy expression in EOM-CCSD(2) using wavefunctions from CIS(D-infinity). Its performance on singlet and triplet valence and Rydberg excitation energies is investigated by comparing with CC3 results. Excitation energies with this method are always overcorrected. On the other hand, reasonable valence and Rydberg excitation energies will be achieved when the energy correction is halved. This method has a non-iterative N-6 scaling and can serve as an economical method in studying both valence and Rydberg excitations.