  We use methods of approximation theory to find the absolute minima on the
sphere of the potential of spherical $(2m-3)$-designs with a non-trivial index
$2m$ that are contained in a union of $m$ parallel hyperplanes, $m\geq 2$,
whose locations satisfy certain additional assumptions. The interaction between
points is described by a function of the dot product, which has positive
derivatives of orders $2m-2$, $2m-1$, and $2m$. This includes the case of the
classical Coulomb, Riesz, and logarithmic potentials as well as a completely
monotone potential of the distance squared. We illustrate this result by
showing that the absolute minimum of the potential of the set of vertices of
the icosahedron on the unit sphere $S^2$ in $\mathbb R^3$ is attained at the
vertices of the dual dodecahedron and the one for the set of vertices of the
dodecahedron is attained at the vertices of the dual icosahedron. The absolute
minimum of the potential of the configuration of $240$ minimal vectors of $E_8$
root lattice normalized to lie on the unit sphere $S^7$ in $\mathbb R^8$ is
attained at a set of $2160$ points on $S^7$ which we describe.
