Long-range first-order interaction energy between two excited hydrogen atoms in either 2s or 2p states.

The long‐range interactions between two hydrogen atoms in either 2s or 2p states yields the first‐order perturbation energy matrix which is still nondiagonal even after the symmetry adaptation of the zero‐order degenerate functions. In the diagonalization process of the nondiagonal blocks, it is found that the familiar series expansion of the interaction potential energy in inverse powers of the internuclear separation is not practicable even up to 60 Bohr units.