= Lennard-Jones potential
{c}
{title2=$U(r)=4\epsilon[(\sigma/r)^{12}-(\sigma/r)^6]$}
{wiki}
Named after <John Lennard-Jones>, this is a pair <potential energy> combining steep short-range repulsion with nonretarded <London dispersion force> attraction. Its minimum is $U=-\epsilon$ at $r=2^{1/6}\sigma$, found by differentiation. The power twelve is a convenient model for repulsion, rather than an exact consequence of the <Pauli exclusion principle>. The power six in the attractive energy follows from second-order coupling of atomic <electric dipoles>, before retardation matters.
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