# Laplace expansion (potential)

In physics, the **Laplace expansion** of a 1/*r* - type potential is applied to expand Newton's gravitational potential or Coulomb's electrostatic potential. In quantum mechanical calculations on atoms the expansion is used in the evaluation of integrals of the interelectronic repulsion.

The Laplace expansion is in fact the expansion of the inverse distance between two points. Let the points have position vectors **r** and **r'**, then the Laplace expansion is

Here **r** has the spherical polar coordinates (*r*, *θ*, *φ*) and **r'** has (*r*′, *θ*′, *φ*′) with homogeneous polynomials of degree *ℓ* in (*x*,*y*,*z*). Further *r*_{<} is min(*r*, *r*′) and *r*_{>} is max(*r*, *r*′). The function is a normalized spherical harmonic function. The expansion takes a simpler form when written in terms of solid harmonics,

## Derivation

One writes

We find here the generating function of the Legendre polynomials :

Use of the spherical harmonic addition theorem

gives the desired result.

## References

- Griffiths, David J. (David Jeffery). Introduction to Electrodynamics. Englewood Cliffs, N.J.: Prentice-Hall, 1981.