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.