An ab initio molecular orbital study of the argon hydride molecule-ions, ArH+ and ArD+, at the MP4(SDQ)/6-311++G(3df,3dp) level. II. The changes in bonding MOs during the Born–Oppenheimer stepwise scan of the internuclear potential energy of the X1Σ+ grou

An analysis of the shapes of the MOs obtained during a Born–Oppenheimer scan of the PE of ArH+ by MP4/6-311++G(3df,3dp) MO calculations, allows the successive disengagement of bonding orbitals to be determined. This clearly shows how the different MOs influence the vibrational–rotational properties of ArH+ and underlines the importance of the concept of orbital following for molecular spectroscopy and the understanding of chemical bonds in molecules. From this follows an explanation why the first excited state of ArH+ and the ground state of ArH are unstable.