The Langevin interaction in molecules

Because of the very weak binding field and this complexity of detail, they pose a formidable challenge to theorists accurate predictions of their properties require an excellent understanding of electron-electron correlations, and computational methods extending beyond the customary basis normally, negative ions do not exist within the Hartree-Fock approximation, precisely because polarisation of the core is not allowed for. 

Our purpose in mentioning negative ions here is to contrast their properties with those of systems which do exhibit Rydberg states. In particular, for negative ions, short pulse excitation is unable to produce narrow wavepackets of the kind described above, which arise by superposition of several Rydberg states. Thus, the response of negative ions to short pulses is markedly different from that of neutral atoms or positive ions, a matter which will be taken up again in chapter 9. 

Readers interested in negative ions may refer to the excellent review by Andersen [66]. 

Most molecules and molecular ions are held together by forces in which electron-electron correlations and exchange interactions play a decisive role. An exception is the hydrogen positive ion Hj, which contains only one electron but is a very stable system. It provides us with an interesting example of how a shallow well can occur near the dissociative limit under conditions reminiscent of those involved in the formation of negative ions. 

In the elementary theory of H2, it is considered as a simple system in which vibrational, electronic and rotational motions can be separated (the Born-Oppenheimer principle) and fully analytic solutions exist (uniquely for a molecule) which show that the molecule is stable. This, however, is not the complete story. In fact, as is separated into H and H+, one encounters an additional shallow minimum near the dissociation limit, at much larger internuclear distances than its equilibrium separation. This second minimum, which arises from a dipole in the neutral fragment induced by the presence of the charged fragment, is capable of supporting 

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