Textbook in PDF format It is the dream of every natural scientist to directly observe the motion of atoms and molecules in real time as they interact. The natural sciences by definition involve structural dynamics and the observation of molecules interconverting from one structure to another or other structures, which is, after all, the very essence of science. Each area of the natural sciences has unique intellectual goals; however, they are all united by the thought experiment in which we try to understand how molecules interconvert between different structures by passing through a reaction barrier, a transition state