This equation is only valid in the sense of distributions, since g ( r ) is not normalized: lim r → → ∞ ∞ g ( r ) = 1 , so that ∫ ∫ V d r g ( r ) diverges as the volume V , leading to a Dirac peak at the origin for the structure factor. Since this contribution is inaccessible experimentally we can subtract it from the equation above and redefine the structure factor as a regular function: