The ab initio mol. dynamics method developed by Car and Parrinello combines an electronic structure method based on d. functional theory (DFT) with a classical mol. dynamics scheme. This provides a powerful, parameter-free tool to study the dynamics of mol. systems in gas or condensed phases. Nowadays, it has become feasible to simulate finite temp. properties or follow chem. reactions for systems of a few hundreds of atoms. The system size can be further increased through the application of mixed QM/MM hybrid techniques. However, one of the most serious remaining limitations is the relatively short time scale of a few tens of picoseconds that can be covered. We have tested different schemes based on bias potentials or forces to extend this restrictive time limits and make the approach more efficient in the study of rare events. [on SciFinder (R)]