An elongation method to calculate the electronic structure of non-periodical and periodical polymers

To calculate the electronic structure of non-periodical polymers, we developed a novel molecular orbital method, which we call the elongation method. When this new method is incorporated into existing molecular orbital methods, such as ab intio molecular orbital method, the computation time decreases substantially, without sacrificing reliability. This new method has two steps, localization and elongation, and thus simulates the process of polymerization reaction. First, we briefly describe the localization and elongation steps. Then, we describe the results from applying the ab initio elongation molecular orbital method (a combination of the two methods) to three poly-hydrogen bonding systems, (H₂O)_n, (HF)_n, and (HCONH₂)n. For all three systems, there was excellent agreement in obtained total energy and electron density compared with those calculated using the conventional ab initio molecular orbital method. Finally, this elongation method was incorporated into the PM3 method and used to calculate the electronic structure of the physiologically active oligopeptide, galanine with 30 amino acid residues. Again, the obtained total energy and the electron density agreed with those calculated by the conventional PM3 method. These results confirm that the elongation method is reliable and can be applied to a variety of huge systems such as proteins and nucleic acids by using various types of molecular orbital theory.