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The equation of motion coupled cluster singles and doubles model (EOM-CCSD)is an accurate, black-box correlated electronic structure approach toinvestigate electronically excited states and electron attachment or detachmentprocesses. It has also served as a basis for developing less computationallyexpensive approximate models such as partitioned EOM-CCSD (P-EOM-CCSD), thesecond-order many-body perturbation theory EOM (EOM-MBPT(2)), and theircombination (P-EOM-MBPT(2)) [S. Gwaltney et al., Chem. Phys. Lett. 248, 189-198(1996)]. In this work we outline an implementation of these approximations forfour-component based Hamiltonians and investigate their accuracy relative toEOM-CCSD for valence excitations, valence and core ionizations and electronattachment, and this for a number of systems of atmospheric or astrophysicalinterest containing elements across the periodic table. We have found thatacross the different systems and electronic states of different natureconsidered, partition EOM-CCSD yields results with the largest deviations fromthe reference, whereas second-order based approaches tend show a generallybetter agreement with EOM-CCSD. We trace this behavior to the imbalance broughtabout by the removal of excited state relaxation in the partition approaches,with respect to degree of electron correlation recovered.

The performance of approximate equation of motion coupled cluster for valence and core states of heavy element systems