Geometry Optimization and Quantum Chemistry

The relevant conformations have now been determined and for each of these conformations the chemical shifts are required. Quantum chemical calculations of an extended polymer chain like in Fig. 15.b are impossible in practice because of the computer time needed. Thus we have to find a small model molecule for quantum chemistry calculations (Fig. 15.c). For the selection of this model molecule, two contradicting requirements must be balanced. 

The necessary computer time increases with the 3 to the 4 power of the size of the atomic basis, which is approximately proportional to the number of non-hydrogen nuclei. For a fast calculation, a small molecule is needed. 

As a simulation of a macromolecule is intended, the central part of the model molecule (denoted by the atoms A, Bi, B2, Ci, C2 in Fig. 15.c) should be representative for the respective sites in a long chain molecule as far as 

The geometries themselves are generated by a force field minimization with the widespread Consistent Valence Force Field (CVFF) (Sect. 2.3) of Biosym Inc. [33]. The dihedral angles are forced to their selected values t9/ by an additional potential 

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