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I read somewhere that the energy is around $U=-\frac{3\alpha I}{4r^6}$ where $\alpha$ is the polarizability and $I$ is the ionization energy but I'm obtaining way too large energies for a lithium crystal.

The shortest distance between two lithium atoms in the crystal is $304 pm$

The first ionization energy of lithium is $520.2 kJ/mol$

I've found various polarizability constants online in papers but I can't really figure them out or the units they are using.

Also, I suspect that I want the dispersion force between $\ce{Li^+}$ ions and not neutral $\ce{Li}$ atoms.

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Polarisability as used here has dimensions of volume. The London formula is $$U \approx -\frac{3}2\frac{I_aI_b}{I_a+I_b}\frac{\alpha _a \alpha _b}{r^6}$$ for two species a and b so you just need to square $\alpha$.

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  • $\begingroup$ So, for a polarizability constant of $24\times 10^{-30}$ which I'm not sure is correct I got a total energy of 290kJ/mol which still seems too high. Is this correct? $\endgroup$
    – Molly Stewart-Gallus
    Jul 25, 2016 at 22:22
  • $\begingroup$ still seems very high $\endgroup$
    – porphyrin
    Jul 26, 2016 at 5:58
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    $\begingroup$ having looked at some values the polarisability of 24 seems very large, atoms seem to have values not bigger than 10 although I have not done a thorough check on this. $\endgroup$
    – porphyrin
    Jul 26, 2016 at 8:45
  • $\begingroup$ The source at arxiv.org/abs/quant-ph/0506106 says about 24 and the source says other sources say 22 and 24. Maybe they mean a different kind of polarizability? $\endgroup$
    – Molly Stewart-Gallus
    Jul 26, 2016 at 19:01
  • $\begingroup$ Volume polarisability is correct. I had a look again and found values for benzene of 25 .10$^{-24}$ ml so that is consistent with your values in m$^3$. Carbon tetrachloride has a value 10.5 for example. ( I had only looked at nitrogen (1.73) and water (1.44), before, apologies). An interaction energy of approx 24000 cm$^{-1}$ is still huge, its as large as an electronic excitation, but I can't see what wrong :( The ionisation energy and bond length seem ok . $\endgroup$
    – porphyrin
    Jul 27, 2016 at 10:59

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