an old puzzle

In the preceding pages, it became clear that the simple mental picture that many of us have about the ionic bond in rocksalt is not in agreement with what DFT tells us. We used to learn that an electron is transferred from Na to Cl to give both of them a complete shell, after which Coulomb attraction between these two ions leads to a strong chemical bond. In the task you were give a few weeks ago, many of you confirmed this picture. However, counting the charges as we did on the previous page does not point to such an electron transfer. Nor do the charge density difference plots. Let’s examine what is going on.

To prepare yourself for the answer, google first some information (and a picture) about bonding and antibonding orbitals. Within a few minutes, you will know why a simple diatomic molecule (such as H₂) is hold together. Once you understood or refreshed this concept, you are ready for this video about crystal orbital densities and rocksalt bonding (they rely heavily on concepts from the previous section on bandstructure, DOS and plotting orbitals and densities – you did not learn, however, how to draw the single particle orbital for a given k-vector that are shown in this video):