Direct calculation of excitation energies by the coupled‐cluster method: Mg and Ar atoms

An open‐shell coupled‐cluster method for the direct calculation of excitation energies is presented and applied to excitations of atomic Mg and Ar. The so‐called CCSD + T approximation involves the exact inclusion of T₁ and T₂ operators and the lowest‐order inclusion of T₃. Quasi‐complete model spaces, built from determinants with one valence hole and one valence particle, are used. Two ionization potentials and 20 excitation energies are calculated. Good agreement with experiment (˜0.2 eV) is obtained. The T₃ effects are less important than for the Be and Ne atoms.