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Problem with Heun's Method

  1. Feb 8, 2016 #1
    1. The problem statement, all variables and given/known data
    Use the Heun method to compute the period of small oscillations about the equilibrium position of a nitrogen atom.
    xi = 1.1
    Um = 7.37
    x0 = 1.2
    alpha = 2.287
    m = 2.325e-26

    2. Relevant equations

    U(x) = Um((1-e^(-alpha(x-x0)))^2 - 1)

    3. The attempt at a solution

    I was told to take the derivative of dU/dx and got (-1/m)*(2*Um*alpha*(1-e**(-alpha*(x-x0)))*(e**(-alpha*(x-x0)))) and used that to calculate my heun step with a time step of 0.5e-15. With this I get a graph that looks like this: 1XxRKQK.jpg which is a velocity vs. time graph. I'm wondering if this graph looks correct and if so why? I thought it would turn out to be something more like: DampedSine.png

  2. jcsd
  3. Feb 8, 2016 #2


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    If ##U(x) = U_m((1-e^{(-alpha(x-x0)))^2 - 1)}##, it's not clear how you calculated

    $$U'(x) = (-1/m)*(2*Um*alpha*(1-e^{(-alpha*(x-x0)))}*(e^{(-alpha*(x-x0)))}))$$

    I'm assuming that ##U_m## is a constant and not ##U ⋅ m##.

    You might want to use some substitutions in the original expression for U(x) to simplify its form before taking the derivative. I would check the derivative using a tool like Wolfram Alpha before writing code with it.
  4. Feb 8, 2016 #3
    Yea sorry about my bad formatting you're right it is a constant and I used python to compute the derivative.
  5. Feb 8, 2016 #4


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    I'm not sure how you do that, but I would check the derivative which Python gave against a pencil and paper derivative or Wolfram Alpha.
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