The Dineon Pair Potential

Electron correlation studies demand basis sets that are capable of very high accuracy, and the 6-31IG set I used for the examples above is not truly adequate. A number of basis sets have been carefully designed for correlation studies, for example the correlation consistent basis sets of Dunning. These go by the acronyms cc-pVDZ, cc-pVTZ, cc-pVQZ, cc-pV5Z and cc-pV6Z (double, triple, quadruple, quintuple and sextuple-zeta respectively). They include polarization functions by definition, and (for example) the cc-pV6Z set consists of 8. 6p, 4d, 3f, 2g and Ih basis functions. 

I have included some representative results in the table, using the cc-pVTZ basis set. 

If the neon-neon interaction were a pure dispersion one, then the HF-LCAO calculation would give a fully repulsive curve. The fact that the HF-LCAO calculation gives a shallow minimum implies an element of covalency. 

Most of the calculations cited give qualitative agreement with experiment , but none agrees quantitatively. 

HF/6-311G MP2/6-311G CISD/6-311G HF/cc-pVTZ MP2/cc-pVTZ CISD/cc-pVTZ Lennard-Jones (I) Maitland, et al. (1981) Lennard-Jones (II) 

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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. 

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