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Quantum chemistry

2007 Schools Wikipedia Selection. Related subjects: General Chemistry

          This article is a historical introduction to the theoretical
          concepts of quantum chemistry. For information on computational
          methods in chemistry and more recent and/or technical aspects of
          quantum chemistry, see computational chemistry. For theoretical
          concepts related to chemistry see theoretical chemistry.

   Linus Pauling, as a pioneer of the valence bond theory, is one of the
   first quantum chemists. He received his first Nobel prize in 1954.
   Enlarge
   Linus Pauling, as a pioneer of the valence bond theory, is one of the
   first quantum chemists. He received his first Nobel prize in 1954.

   Quantum chemistry is a branch of theoretical chemistry, which applies
   quantum mechanics and quantum field theory to address issues and
   problems in chemistry. The description of the electronic behaviour of
   atoms and molecules as pertaining to their reactivity is one of the
   applications of quantum chemistry. Quantum chemistry lies on the border
   between chemistry and physics, and significant contributions have been
   made by scientists from both fields. It has a strong and active overlap
   with the field of atomic physics and molecular physics, as well as
   physical chemistry.

Electronic structure

   The first step in solving a quantum chemical problem is usually solving
   the Schrödinger equation (or Dirac equation in relativistic quantum
   chemistry) with the electronic molecular Hamiltonian. This is called
   determining the electronic structure of the molecule. It can be said
   that the electronic structure of a molecule or crystal implies
   essentially its chemical properties.

Wave model

   The foundation of quantum mechanics and quantum chemistry is the wave
   model, in which the atom is a small, dense, positively charged nucleus
   surrounded by electrons. Unlike the earlier Bohr model of the atom,
   however, the wave model describes electrons as " clouds" moving in
   orbitals, and their positions are represented by probability
   distributions rather than discrete points. The strength of this model
   lies in its predictive power. Specifically, it predicts the pattern of
   chemically similar elements found in the periodic table. The wave model
   is so named because electrons exhibit properties (such as interference)
   traditionally associated with waves. See wave-particle duality.

Valence bond

   Although the mathematical basis of quantum chemistry had been laid by
   Schrödinger in 1926, it is generally accepted that the first true
   calculation in quantum chemistry was that of the German physicists
   Walter Heitler and Fritz London on the hydrogen (H[2]) molecule in
   1927. Heitler and London's method was extended by the American
   theoretical physicist John C. Slater and the American theoretical
   chemist Linus Pauling to become the Valence-Bond (VB) [or
   Heitler-London-Slater-Pauling (HLSP)] method. In this method, attention
   is primarily devoted to the pairwise interactions between atoms, and
   this method therefore correlates closely with classical chemists'
   drawings of bonds.

Molecular orbital

   An alternative approach was developed in 1929 by Friedrich Hund and
   Robert S. Mulliken, in which electrons are described by mathematical
   functions delocalized over an entire molecule. The Hund-Mulliken
   approach or molecular orbital (MO) method is less intuitive to
   chemists, but has turned out capable of predicting spectroscopic
   properties better than the VB method. This approach is the conceptional
   basis of the Hartree-Fock method and further post Hartree-Fock methods.

Density functional theory

   The Thomas-Fermi model was developed independently by Thomas and Fermi
   in 1927. This was the first attempt to describe many-electron systems
   on the basis of electronic density instead of wave functions, although
   it was not very successful in the treatment of entire molecules. The
   method did provide the basis for what is now known as density
   functional theory. Though this method is less developed than post
   Hartree-Fock methods, its lower computational requirements allow it to
   tackle larger polyatomic molecules and even macromolecules, which has
   made it the most used method in computational chemistry at present.

Chemical dynamics

   A further step can consist of solving the Schrödinger equation with the
   total molecular Hamiltonian in order to study the motion of molecules.
   Direct solution of the Schrödinger equation is called quantum molecular
   dynamics, within the semiclassical approximation semiclassical
   molecular dynamics, and within the classical mechanics framework
   molecular dynamics (MD). Statistical approaches, using for example
   Monte Carlo methods, are also possible.

Adiabatic chemical dynamics

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

Non-adiabatic chemical dynamics

   Non-adiabatic dynamics consists of taking the interaction between
   several coupled potential energy surface (corresponding to different
   electronic quantum states of the molecule). The coupling terms are
   called vibronic couplings. The 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 the
   transition probability between two diabatic potential curves in the
   neighbourhood of an avoided crossing to be calculated.

Quantum chemistry and quantum field theory

   The application of quantum field theory (QFT) to chemical systems and
   theories has become increasingly common in the modern physical
   sciences. One of the first and most fundamentally explicit appearances
   of this is seen in the theory of the photomagneton. In this system,
   plasmas, which are ubiquitous in both physics and chemistry, are
   studied in order to determine the basic quantization of the underlying
   bosonic field. However, quantum field theory is of interest in many
   fields of chemistry, including: nuclear chemistry, astrochemistry,
   sonochemistry, and quantum hydrodynamics. Field theoretic methods have
   also been critical in developing the ab initio Effective Hamiltonian
   theory of semi-empirical pi-electron methods.

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