EOM-CC methods with approximate triple excitations applied to core excitation and ionisation energies

A number of iterative and perturbative approximations to the full equation-of-motion coupled cluster method with single, double, and triple excitations (EOM-CCSDT) are evaluated in the context of calculating the K-edge core-excitation and core-ionisation energies of several small molecules. Several of these methods are found to accurately reproduce the full EOM-CCSDT energies well, in particular the EOM-CCSD* method which, when using the core-valence separation (CVS) ansatz, scales rigorously with the sixth power of molecular size. These results highlight that very accurate core excitation and ionisation energies for molecules with first-row atoms can be computed at a cost not much larger than that for EOM-CCSD.