The branch of chemistry that deals with the interpretation of chemical phenomena and properties in terms of the underlying physical processes, and with the development of techniques for their investigation. Chemical physics refers to a branch of physical chemistry in which the emphasis is on the interpretation and analysis of the physical properties of individual molecules and bulk systems, instead of their reactions. Theoretical chemistry is another major branch, in which the emphasis is on the calculation of the properties of molecules and systems, and which uses the techniques of quantum mechanics and statistical thermodynamics. For the present purpose it is convenient to regard physical chemistry as dealing with three aspects of matter: its equilibrium properties, the structure of molecules and bulk matter, and matter's ability to change.