  The lowest-lying states of LiH have been widely used to develop and calibrate
many different methods in quantum mechanics. In this paper we show that the
electron-transfer processes occurring in these two states are a difficult test
for chemical bonding descriptors and can be used to assess new bonding
descriptors on its ability to recognize the harpoon mechanism. To this aim, we
study the bond formation mechanism in a series of diatomic molecules. In all
studied electron-reorganization mechanisms, the maximal electron-transfer
variation point along the bond formation path occurs when about half electron
has been transferred from one atom to another. If the process takes places
through a harpoon mechanism, this point of the reaction path coincides with the
avoided crossing. The electron sharing indices and one-dimensional plots of the
electron localization function and the Laplacian of the electron density along
the molecular axis can be used to monitor the bond formation in diatomics and
provide a distinction between the harpoon mechanism and a regular
electron-reorganization process.
