Separated-atom energy

In calculating a pair potential for dineon, we have to take the separated atom energy as one half of the pair energy for an arbitrarily large distance. 

If the interaction between two ground-state He atoms were strictly repulsive (as predicted by MO theory), the atoms in He gas would not attract one another at all and the gas would never liquefy. Of course, helium gas can be liquefied. Configuration-interaction calculations and direct experimental evidence from scattering experiments show that as two He atoms approach each other there is an initial weak attraction, with the potential energy reaching a minimum at 3.0 A of 0.0009 eV below the separated-atoms energy. At distances less than 3.0 A, the force becomes increasingly repulsive because of overlap of the electron probability densities. The initial attraction (called a London or dispersion force) results from instantaneous correlation between the motions of the electrons in one atom and the motions of the electrons in the second atom. Therefore, a calculation that includes electron correlation is needed to deal with dispersion attractions. 

Figure 7-7 Separated atom energies and energies at an intermediate R for hJ.

Production and Separation. (Atomic Energy Establishment, Cairo (Egypt)). 1971. 32 p. Dep. NTIS. 26 53056... 

The fact that the separated-atom and united-atom limits involve several crossings in the OCD can be used to explain barriers in the potential energy curves of such diatomic molecules which occur at short intemuclear distances. It should be noted that the Silicon... 

Working areas within certain industrial buildings may have restricted access (i.e. sterile laboratories, radioactive areas). Separate toilet accommodation may be required in these areas, an assessment of which may be obtained from the operator and reference to such publications as Atomic Energy Code of Practice, Laboratory Practice. It may also be necessary to operate such appliances remotely by photoelectric cell, sonic control or foot control. 

Since energy is absorbed in reaction (7), the molecule H2 is more stable (has a lower energy) than two separated atoms. This chemical bond (and every chemical bond) forms because the energy is lower when the atoms are near each other. 

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