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Expansion of a point-charge potential

  1. Aug 20, 2010 #1
    I'm currently trying to manipulate the expansion

    [tex]\frac{1}{|x_1-x_2|} = \sum_{l=0} \frac{r_{<}^l}{r_{>}^{l+1}}P_{l}\left(cos\left(\gamma\right)\right)[/tex]

    Where r< is the lesser of |x1| and |x2| and P_l are the legendre polynomials.

    I have included up to 4 terms and the resultant potential isn't resembling the expected potential curve. Does anyone have any experience with such expansions, or would you recommend a different expansion (I ultimately need it to make a function integrable) ?
  2. jcsd
  3. Aug 20, 2010 #2


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    What is the resultant potential you are getting, and what were you expecting? What exactly are you trying to make integrable?
  4. Aug 21, 2010 #3


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    Here you go:
    Carlson and Rushbrooke, "On the expansion of a coulomb potential in spherical harmonics"
    Mathematical Proceedings of the Cambridge Philosophical Society (1950), 46: 626-633

    Equations Ia and Ib give you the correct expansion.
  5. Aug 26, 2010 #4
    Hi, thanks. I have implemented Ib and it seems to have made an improvement.

    gabbagabbahey: When considering |x1-X|^-1: If X is close to the origin and on the z axis, I get a predictable approximation to |x1-X|^-1 (i.e. If I plot the approximate potential across the singularity, I get a finite peak). However, if X is placed further away from the origin, the potential changes, and a caldera forms. I ultimately am constructing plane-wave molecular integrals, so perhaps this isn't even the best approach.
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