Geometry optimization numerical examples

The literature listed in Table 2 gives numerous examples of calculated bond lengths and angles with optimized geometry for the tetrazole tautomers. The electronic structures of the substituted 4,5-dihydrotetrazoles (14) have been investigated by MNDO and AM 1 calculations to explain their photoelectron spectra <88CB1213>. Their electronic structure resembles that of the c -2-tetrazene group and is characterized by three occupied n molecular orbitals and two n molecular... 

Ab initio HF calculations and geometry optimizations on moderate-size molecules have become routine, and computationally efficient methods (for example, DFT and MP2) for inclusion of electron correlation are available. The degree of reliability of various quantum-mechanical methods and basis sets has been established by numerous calculations. The size of a molecule for which one can do an accurate ab initio or density-functional calculation is limited by the speed and storage capacity of... 

The numerical aspects of solving the Poisson or Poisson-Boltzmann equations make them too demanding for use in connections with for example geometry optimizations of macromolecules. For certain special cases, however, the Poisson equation (14.52) can be solved analytical, and this forms the basis for many approximate models for estimating the electronic component in eq. (14.49). 

Finally, we note that Jkl is very sensitive to variations in the molecular geometry. It has been repeatedly observed that the best results are obtained when the same approximation (for instance DFT functional and basis set) is used for geometry optimization and for the following calculation of spin-spin coupling constants. We refer to the recent reviews discussing the calculation of spin-spin coupling constants (Aucar 2008 Helgaker et al. 2008 Krivdin and Contreras 2007) for a discussion of many practical issues and numerous examples of successful applications. 

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