An efficient cluster elongation method in density functional theory and its application to poly-hydrogen-bonding molecules

The elongation method to synthesize the electronic states of polymers is developed at the level of the density functional theory using the linear combination of Gaussian-type orbitals local spin density method. In this treatment, the interactions between the localized molecular orbitals of a cluster and the canonical molecular orbitals of an attacking monomer are successively included, where the Kohn-Sham equation is self-consistently solved instead of the Hartree-Fock equation in the conventional ab initio method. In the process of the cluster extension, an efficient treatment is implemented to calculate the matrix elements of Coulomb integral and exchange-correlation potential. The reliability and the efficiency of this method are examined via applications to hydrogen molecule cluster, linear water cluster (H ₂O)_n and formamide cluster (CHONHH₂) _n. It was shown that the present method saves significantly the computational time and disk storage in the large cluster calculations, and provides good agreements with the results by the conventional density functional treatment for the whole system.