Potential energy surfaces, calculation separated pairs

Figure 2.8 is the energy schematic of the combined system. As the tip and the sample approach each other with a finite bias V, the potential 1/ in the barrier region becomes different from the potentials of the free tip and the free sample. To make perturbation calculations, we draw a separation surface between the tip and the sample, then define a pair of subsystems with potential surfaces Us and Un respectively. As we show later on, the exact position of the separation surface is not important. As shown in Fig. 2.8, we define the potentials of the individual systems to satisfy two conditions. First, the sum of the two potentials of the individual systems equals the potential of the combined system, that is. 

Figure 6.8. Variation with separation distance r of the interaction energy due to van der Waals forces as calculated by the pairwise interaction model, between two different atoms (a) and between two surfaces (b). A, the Hamaker constant, equals n2C p,p2 where C is the constant of the atom-atom pair potential in equation (6.11) (case a) and p 1 and p2 are the number of atoms per unit volume in the...

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