Writing a height-dependent pair potential
In this section, we will create a pair style that represents a potential between two atoms, which decays as the height increases from a reference level, so that both the distance between the atoms and the heights of the atoms above the reference determine the potential between them.
A height-dependent potential is relevant when simulating the interaction between atoms that are adsorbed on a substrate, where forces mediated by the substrate vary with the height. As a result, along with the usual inter-molecular interactions, two adsorbed atoms on the substrate experience additional interaction by virtue of their proximity to the substrate. At a certain threshold above the substrate, the interaction decays to zero. An example of such an interaction is the McLachlan interaction (https://doi.org/10.1016/0039-6028(94)91259-9) based on repulsion between dipoles of adsorbed atoms and the substrate.
A model is established where 
represents the...
