code: function() {
      var center = this.center();
      var radiusSquare = 0.0;
      this.eachAtom(function(atom) {
        radiusSquare = Math.max(radiusSquare, vec3.sqrDist(center, atom.pos()));
      });
      return new geom.Sphere(center, Math.sqrt(radiusSquare));
    }
docs: returns a sphere containing all atoms part of this structure. This will not calculate the minimal bounding sphere, just a good-enough approximation.