6. Diffusive and reactive elementary processes

In this section, we will apply the principles of molecular modeling to the calculation of transition barriers and frequency factors.

A chemical reaction, or the diffusion of an atom on a surface, or the migration of a gap in a crystal lattice, correspond to changes in the configuration of atomic nuclei from one stable or metastable state to another. The point representing the system moves from one local minimum to another on the hypersurface of free energy (HSEL). Any path for changing the relative nuclear coordinates (deforming the initial configuration) is possible, but the principle of least action states that the most likely path is the one at any point where the energy is minimal with respect to any direction except the one collinear with it (Intrinsic Reaction Path, IRC). Potential energy maxima along this path are minima in all other directions: they constitute...