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Homework Help: About Lennard Jones potential.

  1. Oct 18, 2015 #1
    1. The problem statement, all variables and given/known data
    I have a fcc structure, the net parametre is 4.46*10-10m, the potential energy/mol is -1.88kJ/mol. My task is to find the parametres ε and σ of the potential.

    2. Relevant equations


    3. The attempt at a solution
    I can find the value of r using the net paramentre; r=[(√2)/4]*a.
    If I divide the potential energy/mol by 6.02*1023 I can find the potential energy/atom, but I still have only one equation with both of the parametres to determine.
    I have to find another equation with one or both paramentres, but I don't know where to find it.

    All sugestions are appreciated.
    Thank you very much.
  2. jcsd
  3. Oct 18, 2015 #2


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    2017 Award

    Staff: Mentor

    You know that this distance r in your material is at a special point of the potential. Otherwise the distance would increase or decrease.
  4. Oct 18, 2015 #3


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    Homework Helper

    There is a mistake in the formula. It should be Φ=-4ε[(σ/r)6-(σ/r)12]
    The neighbouring atoms are arranged at such distance from each other where the L-J potential is minimum. Find the derivative of Φ and make it equal to zero: this gives you an additional equation.
  5. Oct 18, 2015 #4
    Thank you so very much! It's a brilliant idea.
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