Equilibrium geometries thermochemical calculations

Near the equilibrium geometry, dependence of the energy on geometric displacements is approximately quadratic. As a result, small errors in the reference geometry will insignificantly affect computed energies, but more substantial errors (say, several hundredths of an A in covalent bond lengths) will compromise the reliability of a thermochemical calculation. 

The Hartree-Fock approximation also provided the basis for what are now commonly referred to as semi-empirical models. These introduce additional approximations as well as empirical parameters to greatly simplify the calculations, with minimal adverse effect on the results. While this goal has yet to be fully realized, several useful schemes have resulted, including the popular AMI and PM3 models. Semi-empirical models have proven to be successful for the calculation of equilibrium geometries, including the geometries of transition-metal compounds. They are, however, not satisfactory for thermochemical calculations or for conformational assignments. Discussion is provided in Section n. 

While experimental thermochemical data on these reactions are unavailable, in all cases the calculated data show little sensitivity to choice of equilibrium geometry. Use of approximate geometries for such types of comparisons offers the promise of large cost savings with negligible detrimental effect. 

Quantum chemistry is the foundation of molecular chemistry dealing with structure, properties, and interaction of molecules. The basic principles are offered by quantum mechanics. Quantum-chemical calculations are able to supply information needed for molecular descriptors for QSAR analyses. The use of quantum-chemical calculations is becoming common to establish molecular equilibrium geometries and conformations and to supply quantitative thermochemical and kinetic data. 

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