ABSTRACT
One of the most important questions of chemical reactivity is what happens during chemical reactions. This question can be understood in reaction dynamical experiments and calculations simulating them. In the theoretical description, the motion of atoms is followed in time. The motion of atoms depends on what kind of forces act between the atoms at various relative arrangements, which is determined by the potential energy surface (PES) of the reaction. This is a multidimensional surface (hypersurface), giving the energy as a function of the 3N coordinates of the N atoms participating in the reaction. If the conditions ensuring the applicability of the BornOppenheimer approximation are valid, then the forces acting between atoms do not depend on the velocities of the atomic nuclei, and the potential energy is given by the solution of the electronic Schrödinger equation at each molecular arrangement.
