Long‐ and intermediate‐range interaction in three lowest sigma states of the HeH⁺ ion

Accurate variational energies have been calculated for three lowest sigma states of the HeH + ion. This includes the ground state (5 ≤ R ≤ 9 a.u.) which dissociates into He + H + , as well as the A 1 Σ + state (4 ≤ R ≤ 10) and the a 3 Σ + state (3 ≤ R ≤ 10) which both dissociate into He + + H. The variational results are compared with those obtained using a perturbation theory expansion.