More Advanced Treatments of Molecular Electronic Structure. Computational Chemistry

More Advanced Treatments of Molecular Electronic Structure. Computational Chemistry 

Our discussion of molecular electronic structure has been extremely crude compared with current quantum chemistry practice. Over the past several decades modern digital computers have made calculations possible that previously could only be dreamed of. Dewar and Storch have written a review article comparing the results of different semiempirical and ab initio methods in calculating enthalpy changes of reactions. At the time this article was published (1985), no method had given accuracy that is adequate for quantitative chemical purposes for anything but a few small molecules. However, considerable progress has been made since that time. 

The extended Hiickel method was pioneered by Wolfsberg and Helmholz. It is not restricted to the electrons in pi orbitals and can treat molecules that are not planar 

The constant K has been assigned various values from 1 to 3. A value of 1.75 is common. A slightly different version is employed by Ballhausen and Gray, who use the geometric mean instead of the arithmetic mean in approximating 

The extended Hiickel method can be used to find the conformation of lowest energy by repeated calculations with different conformations. Reasonable results are obtained, and the resulting conformations are sometimes used as starting points for more sophisticated calculations. 

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