Quantum chemistry

Quantum chemistry is a branch of theoretical chemistry where quantum mechanics and quantum field theory are applied. It mathematically describes the fundamental behavior of matter on a molecular scale. An application of quantum chemistry is the study of the behavior of atoms and molecules, in terms of their optical, electrical, magnetic and mechanical properties, and also their chemical reactivity, their redox properties, etc., but both extended solid materials and materials are also studied. surfaces.

The study of quantum chemistry has a strong and active relationship with some scientific fields such as molecular physics, atomic physics and physical chemistry, and contributions in this regard come from both physicists and chemists. The one carried out by the German scientists Walter Heitler and Fritz London is often considered the first calculation in quantum chemistry (although Heitler and London are often considered physicists). The Heitler and London method was perfected by American chemists John C. Slater and Linus Pauling, to become the valence bond theory (also called the Heitler-London-Slater-Pauling (HLSP) theory). In this method, Particular attention is paid to the interactions between pairs of atoms, and is therefore closely related to classical bonding schemes between atoms.

Friedrich Hund and Robert S. Mulliken developed an alternative method, the theory of molecular orbitals, in which electrons were described by mathematical functions delocalized throughout the molecule. The Hund-Mulliken (or molecular orbital) method is less intuitive to chemists; however, having proven to be more powerful in predicting properties than the valence bond method, it is virtually the only one used in recent years.^{[citation needed]}

Areas of interest

Some subtopics of interest in quantum chemistry are:

• the approach of Born-Oppenheimer;
• the method of the self-consistent field;
• Hartree-Fock approach;
• the theory of functional density.

Chemical dynamics

One more step can be to solve the Schrödinger equation with the total molecular Hamiltonian to study the movement of molecules. The direct solution of Schrödinger's equation is called quantum molecular dynamics, within the semiclassical approach semiclassical molecular dynamics, and within the framework of classical mechanics molecular dynamics (MD). Statistical approximations are also possible, using, for example, the Monte Carlo method, and mixed quantum-classical dynamics.

Adiabatic Chemical Dynamics

In adiabatic dynamics, interatomic interactions are represented by scalar potentials called potential energy surfaces. This is the Born-Oppenheimer approximation introduced by Born and Oppenheimer in 1927. Pioneering applications of it in chemistry were made by Rice and Ramsperger in 1927 and Kassel in 1928, and generalized into RRKM theory in 1952 by Marcus who took into account the transition state theory developed by Eyring in 1935. These methods allow easy estimates of unimolecular reaction rates from a few potential surface features.

Non-adiabatic chemical dynamics

Non-adiabatic dynamics consists of taking the interaction between several coupled potential energy surfaces (corresponding to different quantum states of the molecule). The coupling terms are called vibronic couplings. Pioneering work in this field was done by Stueckelberg, Landau, and Zener in the 1930s, in their work on what is now known as the Landau-Zener transition. Their formula allows one to calculate the transition probability between two curves of non-adiabatic potential in the vicinity of an avoided crossing. Spin-forbidden reactions are a type of non-adiabatic reaction in which at least one change of spin state occurs when going from reactant to product.

Quantum Chemists

The community of scientists who have made great contributions to quantum chemistry include:

• Erich Hückel
• Friedrich Hund
• Wolfgang Ernst Pauli
• Linus Pauling
• John C. Slater