## Computational Quantum Chemistry

With the rapid performance increase of the available hardware, computers
became an important tool in organic chemistry. Together with recent
advances in quantum mechanical methods, the accurate calculation of
reactants, intermediates and transition structures of a variety of
reactions is now feasible. These computations yield informations that
would be very difficult to obtain otherwise. Based on these results,
predictions about new reactions can be made that in turn inspire new
experiments to test these predictions.

During the last few years, *density functional theory (DFT)*
emerged as a new and promising tool for quantum mechanical studies.
By comparison with experimental data and more established methods such
as *Hartree-Fock theory * and *multiple configuration/complete
active space calculations *, the reliability of DFT methods for
radical ions and other open shell species is investigated.

The appropriate method will then be used to calculate the geometric
and electronic structure of the reactants and transition states
involved in the reaction. In particular, the computation of the
vibronic force constants allows the theoretical prediction of the
*kinetic isotope effects (KIE)*. Comparison of these theoretical
values with the experimentally
determined KIE provides a close link between theoretical and
experimental studies.

### Structure and electrostatic potential map of a model system for
the thymine dimer radical anion