Polymers semiempirical band-structure

The band structure of fully protonated emeraldine salt was studied previously by semiempirical molecular orbital (MO) calculations and more recently by ab initio calculation [41,42]. A half-filled polaron band formed via the interaction between separate polarons (there are two polarons per tetrameric repeating unit) was proposed to explain the observed optical and electrical properties of fully protonated PANI-ES. The PANI-HCSA films used in these studies were, however, cast from NMP solutions in the form of EB and then doped into the form of ES. The polymer chains in these polymer films, therefore, have the same conformational structure. 

After a number of semiempirical (extended Huckel, CNDO/2,< > INDO,< and MINDO< >) calculations in 1970, Andre performed the first ab initio band-structure computation of an infinite polyethylene chain. Since this polymer is mass-produced in the plastics industry, the theoretical study of its properties (especially of its mechanical properties, see Chapter 10) is of great practical interest, which explains the large... 

In contrast to molecular mechanics force fields, modern semiempirical methods are classified as an SCF electron-structure theory (wavefunction-based) method [12]. Older (pre-HF) semiempirical approaches such as extended Hvickel theory, which can be classified as a one-electron effective Hamiltonian approach, involve drastic approximations but rely on the researcher s intuition and ability to extrapolate from simple computations to meaningful chemistry. This method is not used much these days but still plays a role in determining the band structures of organic polymers, most of which are carbon-rich by definition [13]. 

At least two semiempirical methods, MINDO/3 and MNDO, have been applied successfully to the study of linear polymers using conventional solid-state theoretical techniques. The MINDO/3 calculation, showed how the band-structure of polyethylene could be calculated, while in the MNDO calculation, the optimized geometry, electronic band structure, and vibrational frequencies for polyethylene were calculated. These calculation used conventional methods, which rely on the factorization of the infinite Hamiltonian into complex symmetry adapted functions, followed by the use of those functions in the construction of a real density matrix. A more general solid-state method has been developed, but like the other conventional methods, it is very slow, and these methods have not been used to any great extent. 

Due to the central role of DNA and proteins in biochemistry and biophysics the computation of the electronic structure of periodic polymers built from nucleotide bases, base pairs, nucleotides and amino acids, respectively, had been of high interest since about twenty years. Early calculations of the band structure of DNA related periodic polymers have been performed with the crystal orbital (CO) method on the basis of different semiempirical levels (1). Recently the results of ab initio Hartree-Fock CO (2, 3) band structure calculations for the four nucleotide base stacks (4-6), the two Watson-Crick base pair stacks (6), the sugar-phosphate chain (4,5) and the three nucleotides cytidine (4,5), adenylic acid and th3nnidine (6) have been reported. These computations represent a significant progress but the following improvements are required for a more accurate description of the electronic structure of real DNA and its transport properties ... 

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