A common and useful parameter for the explanation of the compositional dependence on the physical properties is the coordination number Z of covalent bonds per atom, characterizing the structural atomic units [5,13,14].

We can consider such a scenario by choosing as the initial state a tightly localized state given by [psi](x, 0) = [square root of (2)] sin [k.sub.0]x, with [k.sub.0] = [pi] and the barrier parameters a = 1, b = 2, and [V.sub.0] = 11, in Rydberg atomic units. In this situation the initial wave function is perfectly confined to the left of the barrier (0 < x < 1), and above-the-barrier components contribute in a significant way to build the wave packet, as can be seen from [rho](k) in the top plot of Figure 8 (in this case the probability of finding a nontunneling k component in the wave packet is approximately 75%).