# Laplacian of the electron density

Is it always the sign of the Laplacian of the electron density a good criterion to decide on the covalent nature of a bond?

Just as in bonding in general, i.e. there is a spectrum from pure covalent (ideally, but not achievable) and pure ionic (ditto),* there cannot be a clear yes or no answer here, too. I think the case of $\ce{BCl3}$ versus $\ce{AlCl3}$ and respective dimers, which I have written about, demonstrates this feature and how QTAIM can help understand bonding. As with many things it certainly is hard to draw a line. In that sense it is probably a good criterion to judge the nature of a bond. Judging only based on the sign of the Laplacian, while being a rigorous and unambiguous way to look at things, might lead to the wrong conclusions though.
• Forgive me my incompetence but not even $\ce{H2}$ can be considered a purely covalent bond? – Jan Nov 1 '17 at 14:40