Symmetric H3: A Semiempirical and Ab Initio Study of a Simple Jahn–Teller System

Semiempirical and multiconfiguration ab initio results for the potential energy of H₃ are used to obtain a detailed description of the potential‐energy surface in the region of interest for the Jahn–Teller problem. Topographical properties of truncated perturbation expansions of the potential‐energy function about the $D_{\text{3h}}$ minimum are compared with those of the complete semiempirical contour map; six symmetric dissociation “troughs” are found in the complete semiempirical surface, in contrast to three stable basins in the second‐order surface and three dissociative troughs in the third‐order surface. The relationships among the various parts of the complete three‐dimensional H₃ potential‐energy hypersurface are discussed and illustrated by means of a single compact diagram. A convenient matrix‐projection method for identifying normal‐mode coordinate and electronic wavefunction partners associated with degenerate representations is discussed in an appendix.