Three regimes of interaction

For convenience, in thi.s. section, atomic units are used, where three constants are set to unity e = h - m = 1. The atomic unit of length is the bohr (0.529 A), and the atomic unit of energy is the hartree (27.21 eV). 

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Fig. 7.1. Three regimes of interaction in the hydrogen molecular ion. (a) At large distances, R>16 a.u., the. system can be considered as a neutral hydrogen atom plus a proton. The polarization of the hydrogen atom due to the field of the proton generates a van der Waals force, (b) At intermediate distances, 16>/ >4 a.u. the electron can tunnel to the vicinity of another proton, and vice versa. A resonance force is generated, which is either attractive or repulsive, (c) At short distances, R<4 a.u., proton-proton repulsion becomes important. (Reproduced from Chen, 1991c, with permission.)...

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