7 Molecular Crystal

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Here is a molecular crystal, with five bands with negative energies. Why is there a pair of closely spaced bands way below the rest of the bands, rather than a single low lying level as we have seen before? First take a look at the wave function psi (the red fan version) which is for a state in the lowest band. Then copy graph the psi-squared plot. Next calculate psi again for the energy -227.9 eV, an energy in the second band. The electron density looks very much like the electron density in the lowest band. Check the red fan (psi) to see the difference.

To explore these two bands in more detail change the energy range of the band structure plot to -232 to -225 and calculate bands Compare the psi-squared plots for states just above and just below the first band gap. Can you find a simple argument for why these states differ in energy?

Open the band list window and leave the energy range the same. What happens to the bands if you change the lattice constant to 2.4 Angstrom? Now change the mol. sep. to 1.1 Angstrom which returns the distance between atoms in neighboring molecules to the original value. Explain why the band edges move in the way they do!