Figure 1.

The potential energy landscape of an atomic pair interaction as a function of both the inter-atomic distance r_{i j} and λ. Both electrostatic and van der Waals soft-core potentials are used. Typical values for atomic partial charges and Lennard-Jones parameters from the Amber forcefield^23,24 have been used. At regions of low λ both the strong van der Waals repulsion at small r_{i j} and the energy minimum at the most favorable interaction distance of approximately 3 Å can be seen. Conversely, at λ-values close to one, the potential energy surface is flat. The problematic intermediate region at λ ≈ 0.3 in which the interplay of softened electrostatic and van der Waals interactions generates a potential curve with two minima (marked A and B) can be distinguished. Values for α_LJ and ${\beta }_C^{1/m}$ of 0.5 and 3.5 Å were used.