The results obtained by this new method are shown to be able to account for most of the shortcomings of the earlier methods, in particular their failure satisfactorily to explain the quenching of luminescence in cis-polyacetylene and their poor predictions of the relative strengths of the two photoinduced absorption peaks in polythiophene. The ability of trans-polyacetylene (t-PA) to support a novel type of dynamic defect known as a breather is also verified. A quantitative estimate is made of the mobility of the fundamental defect in t-PA, known as a soliton, and this is in good agreement with experiment.