Max-Planck-Institut für Festkörperforschung Andersen Group LMTO C60 GW Clusters

## Leonhard Euler (1707-1783)

The Euler characteristic is a number, C, which characterized the various classes of geometric figures. It depends only on the topology, not the specific shape of the figure. For an arbitrary polyhedron it can be calculated from the number of vertices V, edges E, and faces F:

 C = V - E + F (1)

Euler showed that for all simple polyhedra the Euler characteristic is C=2. As an example let's look at a cube: it has V=8 vertices, E=12 edges, and F=6 faces, hence C=8-12+6=2. Do you get the same for, say, a tetrahedron? The hard work, of course, is to show that C is an invariant for all simple polyhedra.

For the special case of a simple polyhedron made from only pentagons and hexagons, the Euler formula (1) simplifies considerably. Let P be the number of pentagons and H be the number of hexagons. Clearly the number of faces is then F=P+H. Each edge is shared by two polygons, so the number of edges is E=(5 P+6 H)/2. The number of vertices is a bit more tricky. There must be at least three polygons sharing a vertex. On the other hand, for a convex polyhedron the sum of the internal angles of the polygons must not exceed 360 degrees. Since the internal angle for a pentagon (hexagon) is 108 (120) degrees, there must not be more than three polygons sharing a vertex. Thus V=(5 P+6 H)/3. Inserting all this into (1) and using C=2, we end up with

12 = P + 0*H

I.e. a simple polyhedron entirely made from pentagons and hexagons must contain exactly 12 pentagons. The number of hexagons can be arbitrary. Given the number of hexagons, we can now easily calculate the number of vertices:

V = 20 + 2 H

But for a fullerene the number of vertices is just the number of carbon atoms in the molecule. We have thus found that fullerenes must have an even number of carbon atoms, and that there is a lower limit for the size of the fullerenes: C20 is the smalles fullerene. It looks like a dodecahedron.