Lagrange-mesh calculations of excited states of three-body atoms and molecules

Energies of S and P bound states of the helium atom He, the hydrogen ion H- , the hydrogen molecular ion H-2(+) and the muonic molecule dt mu are calcu lated with the Lagrange-mesh numerical method. Interparticle distances and mass radii of these states are evaluated very simply with the Gauss quadrat ure approximation associated with the mesh. Except for weakly bound states, highly accurate values are obtained, which agree with or improve published results.