An elongation method, which was already proposed to calculate efficiently the electronic states of aperiodic polymers, is developed to provide the local density of states. Elongation method is carried out by repeating the uniform localization of wave functions with inclusion of the interaction between the cluster and an attacking molecule. At first, we develop this method to the ab initio level. Next, we show that the local density of states (LDOS) is obtained efficiently following the elongation step without treating the whole system directly. The validity of our method is examined via application to a hydrogen molecule cluster by comparing the results with those by the conventional ab initio method for the whole system. This approach makes it possible to investigate the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and their density of states on each site of a polymer chain, which will provide a novel approach to examine intra- and intermolecular reactivities within and between aperiodic polymers.
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