Geometry optimization of polymers by the elongation method

Theoretical studies on the electronic and the geometrical structures for various molecules by the molecular orbital or the density functional theory have recently been developed and applied widely under the progress of computer technologies. At present, it is possible to carry out a theoretical investigation on electronic properties for small molecules at the Hartree-Fock and the post-Hartree-Fock levels by the improvement of advanced program packages. However, it is difficult to perform the theoretical calculations on electronic structures for large polymers with the aperiodic sequence of molecular segments, because the theoretical treatment of random systems has not yet been established. We recently proposed the elongation method as a useful theoretical approach to obtain the electronic states of any polymers without the periodic geometry of molecular fragments. In the previous works, the reliability of our treatment has been shown by the application to many polymers under single-point calculations with fixed molecular geometry. Thus, as the next step of our study, an attempt for the geometry optimization of large polymers by the elongation method was made in this work. As the first samples of geometry optimization, the periodic polymers of polyethylene, polyacetylene, and polyglycine were examined. Also, as the second samples, the locally aperiodic polymers of polyacetylene with local defects of positively and negatively charged solitons were tested. Total energies, optimized geometries, and electron densities were checked by those obtained from the conventional molecular orbital method.