And semiempirical approximations

Ab Initio and Semiempirical Approximations and Modeis. The general formulas of ab initio methods can be used for calculations directly and without approximation, only if the set of wave functions and eigenenergies is known with sufficient accuracy. This is true only in the case of the very simplest atoms and molecules. The from first principles approach is to calculate the wave functions by solving the Schrodinger equation exactly. For large molecules, such as polymer fragments and chains, it is currently impossible to produce a set of exact wave functions or to obtain sufficient spectroscopic data. Therefore various assumptions are introduced to alleviate the complexity of the calculations semiempirical models contain such assumptions—which are beyond the scope of this chapter. However one may rely on semiempirical quantum-mechanical methods to predict (at least qualitatively) properties of polymer chain fragments. 

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