This simple picture would say that polyacetylene is a metallic conductor and its conduction band is half-filled (Fig. 1). However polyacetylene is actually a semiconductor, that is, there is energy gap between the conduction and valence bands. The reason is that the half-filled band sketched on the left of Fig. 1 is correct only if the single and double bonds have the same lengths. The problem is that the equidistant linear chain structure is unstable towards a structural deformation of alternating shorter double and longer single bonds, and the result is a finite band gap