Open‐shell coupled‐cluster method: Electron affinities of Li and Na

The open‐shell coupled‐cluster method and the diagrams needed for its implementation are described. The method is applied to the electron affinities of Li and Na, which are calculated in two ways: as the ionization potential of the anions or as the energy of adding the second electron to the cations. The two schemes give essentially the same results, in very good agreement (<0.02 eV) with experiment. Three‐body effects are negligible.