Here, atoms i and j have two tunable parameters, the equilibrium separation r m,i j and well depth ε i j . The 12-6 potential (n = 12, m = 6) is almost universally used for computational ease, simply because the repulsive 12th power can be obtained by squaring the r -6 term 2 . The simplicity of the LJ potential makes the parameterisation process more tractable and allows for fast computation, but there are limited physical reasons for its widespread use . In particular, although the leading order term in the attractive dispersion interaction decays as r -6 , the LJ potential neglects (or must effectively account for) the higher order terms in the expansion . The r -12 term seeks to model repulsion between electrons, but an exponential decay is more physically justified and provides a more accurate description of repulsion at short distances . A further technical issue with the LJ potential is its divergence as r tends to zero. This is not typically a problem in standard molecular dynamics simulations, but can lead to poor convergence and systematic errors in alchemical free energy calculations as atoms are (dis)appeared. In these cases, the LJ potential must be replaced by a (less simple) soft-core potential , which makes these calculations significantly more difficult and computationally expensive.