where the parameters ε ε 12 and σ σ 12 are related to dispersion strength and particle size respectively, just as in the Mie potential or Lennard-Jones potential, which is the source of the first term, μ μ i is the dipole moment of species i , and ξ ξ is a parameter describing the relative orientation of the two dipoles, which may vary between -2 and 2.