Relativistic two-electron atomic and molecular energies using $LS$ coupling and double groups: role of the triplet contributions to singlet states

Péter Jeszenszki, Edit Mátyus

The triplet contribution is computed to the 1 and 2 ${}^1{S}_0^{\text{e}}$ states of the He atom, to the $1{}^1{S}_0^{\text{e}}$ state of the Li${}^{+}$ and Be${}^{2+}$ ions, and to the $X\ ^1Σ_…