  Considering interatomic potential $U({\mathbf q})$ where ${\mathbf q} =
[{\mathbf q}_1, {\mathbf q}_2, \dots, {\mathbf q}_N] \in {\mathbb R}^{3N}$ is a
vector describing positions, $\mathbf{q}_i \in {\mathbb R}^3$, it is shown that
$U$ can be defined as a function of the interatomic distance variables $r_{ij}
= |{\mathbf q}_i - {\mathbf q}_j |$, provided that the potential $U$ satisfies
some symmetry assumptions. Moreover, the potential $U$ can be defined as a
function of a proper subset of the distance variables $r_{ij}$, provided that
$N > 5$, with the number of distance variables used scaling linearly with the
number of atoms, $N$.
