  An extensive analysis has been carried out of the performance of standard
families of basis sets with the hierarchy of coupled cluster methods CC2, CCSD,
CC3 and CCSDT in computing selected Oxygen, Carbon and Nitrogen K-edge
(vertical) core excitation and ionization energies within a core-valence
separated scheme in the molecules water, ammonia, and carbon monoxide. Complete
basis set limits for the excitation energies have been estimated via different
basis set extrapolation schemes. The importance of scalar relativistic effects
has been established within the spin-free exact two-component theory in its
one-electron variant (SFX2C-1e).
