This paper studies the evolution of the electron densities of two separated
atoms into an equilibrium molecular distribution. A range of interactions
is considered: from closed-shell with and without charge transfer, through
polar-shared, to equally shared interactions. The changes in the density ar
e monitored in terms of the properties of the density at the bond critical
point and the shape of the interatomic surface. The effect of these changes
on the properties of the atoms defined as proper open systems is determine
d. The "harpoon mechanism" operative in the formation of LiF is found to ex
ert dramatic effects on the electron density and on the atomic and molecula
r properties. The virial and the Ehrenfest force theorems in their molecula
r, atomic, and local forms, together with the Hellmann-Feynman theorem, pro
vide an understanding of the similarities and differences in the bonding re
sulting from closed-shell, shared, and polar interactions. The effect of th
e long-range dispersion forces on the electron density and the resulting ch
anges in the kinetic and potential energies, the former decreasing and the
latter increasing on the initial approach of the atoms, are investigated. I
n addition to the changes in the total energy and its kinetic and potential
energy components as a function of the internuclear separation R, the atom
ic contributions to these quantities are also reported. The atomic Ehrenfes
t force is the force acting on the electron density in an atomic basin and
the one measured in an atomic force microscope. It is shown to change from
an intially attractive interaction, to a repulsive one at a separation slig
htly greater than R, where the Hellmann-Feynman forces on the nuclei vanish
.