ABSTRACT
It is often the case that we can consider properties of molecules to be defined by their response to a disturbance. For example, the dipole moment of a molecule is defined by a change in energy of the system as an electric field is applied. Much of spectroscopy is simply the study of responses of a molecular system to a periodic perturbation provided by external fields. Scattering experiments similarly can be considered the study of the effects of molecular collisions, momentary disturbances of the reference systems. Perturbation theory permits a systematic representation of the effects of small disturbances on a reference system, and is the natural language for many molecular phenomena.
