I have read solid state physics which is wrote by kittle. it said that repulsive term is relate with pauli exclusion principle and we can descride it r^-12. but i don`t know why the two cases, for example two hydrogen atoms which have same electron spin or different spin, have the same dependence r^-12. i think that if they are the same, then more repulsive force are acted than the difference one because of pauli explosion principle.

  • $\begingroup$ You should think of the Lennard-Jones as a very crude approximation. There should be more dependencies as you mention, but we are being lazy with how much computation we do. $\endgroup$ – AHusain Aug 23 '16 at 4:53
  • $\begingroup$ Yes, I believe that it is $r^{-12}$ just because it happends to be square of asymptotic scaling of dipolar vdw-fluctuations $r^{-6}$. And square is faster to calculate than arbitary exponent. In paper it makes equilibrium distance solvable with quadratic equation and with computer FPUs it is faster as well. $\endgroup$ – Mikael Kuisma Aug 23 '16 at 5:36

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