Note that for r much greater than σ, the erf function approaches unity, and the potential φ (r) approaches the point-charge potential, φ φ ≈ ≈ 1 4 π π ε ε Q r , as one would expect. Furthermore, the error function approaches 1 extremely quickly as its argument increases; in practice, for r > 3 σ the relative error is smaller than one part in a thousand.