Josh recently investigated different N5 approximations to the traditional N6 scaling EOM-CCSD. The MBPT based variation was consistently better at describing Rydberg excitations while EOM-CC2 was able to resolve valence excitations more accurately. The paper is titled “Assessment of Low-scaling Approximations to the Equation of Motion Coupled-Cluster Singles and Doubles Equations” and is published in The Journal of Chemical Physics.
