The understanding of catalytic and enzymic processes at the mol. level is one of the most challenging problems in chem. and biol. A necessary prerequisite for any theor. modeling of these chem. reactions is an appropriate description of the electronic structure of the reactants. We use ab initio mol. dynamics simulations in the framework of d. functional theory as well as hybrid schemes based on ab initio or semiempirical methods to examine chem. reactions in complex environments. In particular, we have applied these techniques to the characterization of the catalytic reactions of the enzymes human carbonic anhydrase II and HIV protease. [on SciFinder (R)]