Parametrization strategies, semiempirical

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]. 

Originally, there have been two basic strategies for parametrization. Approximate MO methods aim at reproducing ab initio MO calculations with the same minimal basis set (MBS), whereas semiempirical MO methods attempt to reproduce experimental data. Nowadays the limitations of MBS ab initio calculations are well known and the predominant feeling is that approximate MO methods would not be useful enough in practice even if they would exactly mimic MBS ab initio calculations. Hence with the exception of PRDDO [18], current parametrizations usually adhere to the semiempirical philosophy and employ experimental reference data (or possibly, accurate high-level theoretical predictions as substitutes for experimental data see Section III.E). 

The parametrization of the semiempirical treatment outlined [37] employs ground-state reference data and follows the same strategy as in MNDO (see Section II.A). Parameters are available for the elements H, C, N, O, and F. In a statistical evaluation for ground-state properties... 

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