# Theoretical chemistry

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**Theoretical chemistry** is the use of reasoning to explain or predict chemical phenomena. In recent years, it has consisted primarily of quantum chemistry, i.e., the application of quantum mechanics to problems in chemistry. Theoretical chemistry may be broadly divided into electronic structure, dynamics, and statistical mechanics. In the process of solving the problem of predicting chemical reactivities, these may all be invoked to various degrees. Other "miscellaneous" research areas in theoretical chemistry include the mathematical characterization of bulk chemistry in various phases (e.g. the study of chemical kinetics) and the study of the applicability of more recent math developments to the basic areas of study (e.g. for instance the possible application of principles of topology to the study of electronic structure.) The latter area of theoretical chemistry is sometimes referred to as mathematical chemistry.

Much of this may be categorized as computational chemistry, although computational chemistry usually refers to the application of theoretical chemistry in an applied setting, usually with some approximation scheme such as certain types of post Hartree-Fock, Density Functional Theory, semiempirical methods (like for instance PM3) or force field methods. Some chemical theorists apply statistical mechanics to provide a bridge between the microscopic phenomena of the quantum world and the macroscopic bulk properties of systems.

Theoretical attacks on chemical problems go back to the earliest days, but until the formulation of the Schrödinger equation by the Austrian physicist Erwin Schrödinger, the techniques available were rather crude and speculative. Currently, much more sophisticated theoretical attacks based on quantum mechanics are in vogue.

## Branches of theoretical chemistry

- Quantum chemistry
- The application of quantum mechanics to chemistry
- Computational chemistry
- The application of computer codes to chemistry
- Molecular modelling
- Methods for modelling molecular structures without necessarily referring to quantum mechanics. Examples are molecular docking, protein-protein docking, drug design, combinatorial chemistry.
- Molecular dynamics
- Application of classical mechanics for simulating the movement of the nuclei of an assembly of atoms and molecules.
- Molecular mechanics
- Modelling of the intra- and inter-molecular interaction potential energy surfaces via a sum of interaction forces.
- Mathematical chemistry
- Discussion and prediction of the molecular structure using mathematical methods without necessarily referring to quantum mechanics.
- Theoretical chemical kinetics
- Theoretical study of the dynamical systems associated to reactive chemicals and their corresponding differential equations.

Historically, the major field of application of theoretical chemistry has been in the following fields of research:

- Atomic physics: The science of the electrons surrounding the atomic nuclei
- Molecular physics: The science of the electrons surrounding the molecular nuclei and of movement of the nuclei. This term usually refer to the study of molecules made of a few atoms in the gas phase. But some consider that molecular physics is also the study of bulk properties of chemicals in terms of molecules.
- Physical chemistry and chemical physics: Chemistry investigated via physical methods like laser techniques, scanning tunneling microscope, etc. The formal distinction between both fields is that physical chemistry is a branch of chemistry while chemical physics is a branch of physics. In practice this distinction is quite vague.

Hence, the theoretical chemistry discipline is sometimes seen as a branch of those fields of research. Nevertheless, more recently, with the rise of the density functional theory and other methods like molecular mechanics, the range of application has been extended to chemical systems which are relevant to other fields of chemistry and physics like biochemistry, condensed matter physics, nanotechnology or molecular biology.