Semiempirical calculations, polymers

Jalbert, C., Koberstein, J.T., Hariharan, A., Kumar, S.K. End group effects rai surface properties of polymers semiempirical calculations and comparison to experimental surface tensions fora, m-functional poly(dimethylsiloxanes). Macromolecules 30,4481-4490 (1997)... 

Coupled-cluster Theory DNA Bases and Base Pairs Ab Initio Calculations Green s Functions and Propagators for Chemistry M0ller-Plesset Perturbation Theory Polymers Semiempirical Calculations. 

AMI Divide and Conquer for Semiempirical MO Methods Green s Function Ionization Potentials in Semiempirical MO Theory Hydrogen Bonds Semiempirical Methods Localized MO SCF Methods MNDO/d Parameterization of Semiempirical MO Methods PM3 Polymers Semiempirical Calculations Population Analyses for Semiempirical Methods Semiempirical Vibrational Frequencies (Including Scaling). 

While the electronic structure calculations addressed in the preceding Section could in principle be used to construct the potential surfaces that are a prerequisite for dynamical calculations, such a procedure is in practice out of reach for large, extended systems like polymer junctions. At most, semiempirical calculations can be carried out as a function of selected relevant coordinates, see, e.g., the recent analysis of Ref. [44]. To proceed, we therefore resort to a different strategy, by constructing a suitably parametrized electron-phonon Hamiltonian model. This electron-phonon Hamiltonian underlies the two- and three-state diabatic models that are employed below (Secs. 4 and 5). The key ingredients are a lattice model formulated in the basis of localized Wannier functions and localized phonon modes (Sec. 3.1) and the construction of an associated diabatic Hamiltonian in a normal-mode representation (Sec. 3.2) [61]. 

The photochemical model was probed by using one triazene polymer (structure shown in Scheme 5). Semiempirical calculations for a model compound, i.e., a bis-triazene (shown in Scheme 7) were applied to determine a realistic number of chromophores, which results in a decreasing number of free fitting parameters. 

For a more detailed analysis of the absorption properties, the UV spectrum of the model compound (Scheme 7), which was also synthesized, was calculated using semiempirical methods (MOPAC/ZINDO). The experimental UV spectrum of the model compound is nearly identical to the spectrum of the polymer. From the calculation it was derived that four UV transitions contributed to the absorption maximum at 330 nm. In detail, these are the HOMO LUMO, the HOMO->LUMO+l, the HOMO LUMO+2, and the HOMO—LUMO+3 transitions. The first two orbital excitations showed a large involvement of the triazene group, whereas the other two are mainly localized at the phenyl moieties. Similar results were previously reported for aryl dialkyl triazenes [119, 184] which have the same structural unit. Starting from simple chemical considerations, it could be thought that the number of chromophores responsible for the absorbance at around 300 nm is a low value, for example 2 or 4 per unit. On the other hand, the semiempirical calculations indicated the involvement of the phenyl moieties in the absorption properties therefore, the chromophore number in the calculation was not restricted to low values. As a starting point for the calculation, numbers close to the expected value were chosen. 

The valence and conduction bands of PANI are jc-like bands. The orbitals of the lowest unoccupied band have nodes on the nitrogen atoms (as also observed by Libert et al. [66] from VEH calculations on the infinite polymer and by Sjdgren and Stafstrdm [67] fiom semiempirical calculations on a monomer), which breaks the conjugation path. This fact is the main reason for the flatness of the bands closest to the Fermi level. 

The PRDDO (partial retention of diatomic differential overlap) method is an attempt to get the optimal ratio of accuracy to CPU time. It has been parameterized for the periodic elements through Br, including the 3rd row transition metals. It was parameterized to reproduce ah initio results. PRDDO has been used primarily for inorganic compounds, organometallics, solid-state calculations, and polymer modeling. This method has seen less use than other methods of similar accuracy mostly due to the fact that it has not been incorporated into the most widely used semiempirical software. 

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