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For my purpose, now I want to a potential with the form

v= -A/r^6 + B/r^12

It is pretty close to the Lennard-Jones potential in LAMMPS

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It's not just close to the Lennard-Jones potential; it *is* the Lennard-Jones potential. With a little algebra, I got sigma = (B/A)^(1/6) and epsilon = (A^2)/(4B). You may want to check my math, but it shouldn't be too hard to derive expressions where sigma and epsilon are in terms of A and B.