# Quantum chemistry

**quantum chemistquantum chemicalquantum chemical calculationsquantumHistory of quantum chemistryquantum theorycomputational quantum and theoretical chemistryquantum-chemicalapplied theoretical chemistryAtomic Level**

Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems.wikipedia

411 Related Articles

### Born–Oppenheimer approximation

**Born-Oppenheimer approximationBorn–OppenheimerAdiabatic approximation**

Many quantum chemical studies assume the nuclei are at rest (Born–Oppenheimer approximation).

In quantum chemistry and molecular physics, the Born–Oppenheimer (BO) approximation is the assumption that the motion of atomic nuclei and electrons in a molecule can be treated separately.

### Semi-empirical quantum chemistry method

**semi-empiricalSemi-empirical quantum chemistry methodssemi-empirical methods**

On the calculations, quantum chemical studies use also semi-empirical and other methods based on quantum mechanical principles, and deal with time dependent problems.

Semi-empirical quantum chemistry methods are based on the Hartree–Fock formalism, but make many approximations and obtain some parameters from empirical data.

### Linus Pauling

**PaulingLinus Carl PaulingLinus C. Pauling**

In the following years much progress was accomplished by Robert S. Mulliken, Max Born, J. Robert Oppenheimer, Linus Pauling, Erich Hückel, Douglas Hartree, Vladimir Fock, to cite a few.

Pauling was one of the founders of the fields of quantum chemistry and molecular biology.

### Photoelectric effect

**photoelectricphotoelectronphotoemission**

Then, in 1905, to explain the photoelectric effect (1839), i.e., that shining light on certain materials can function to eject electrons from the material, Albert Einstein postulated, based on Planck's quantum hypothesis, that light itself consists of individual quantum particles, which later came to be called photons (1926).

This phenomenon is commonly studied in electronic physics and in fields of chemistry such as quantum chemistry and electrochemistry.

### Relativistic quantum chemistry

**relativistic effectsrelativisticrelativistic effect**

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.

Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to explain elemental properties and structure, especially for the heavier elements of the periodic table.

### Computational chemistry

**computational chemistcomputationalcomputational methods**

Theoretical quantum chemistry, the workings of which also tend to fall under the category of computational chemistry, seeks to calculate the predictions of quantum theory as atoms and molecules can only have discrete energies. This computational affordability and often comparable accuracy to MP2 and CCSD(T) (post-Hartree–Fock methods) has made it one of the most popular methods in computational chemistry.

The programs used in computational chemistry are based on many different quantum-chemical methods that solve the molecular Schrödinger equation associated with the molecular Hamiltonian.

### Quantum mechanics

**quantum physicsquantum mechanicalquantum theory**

Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems.

Important applications of quantum theory include quantum chemistry, quantum optics, quantum computing, superconducting magnets, light-emitting diodes, the optical amplifier and the laser, the transistor and semiconductors such as the microprocessor, medical and research imaging such as magnetic resonance imaging and electron microscopy.

### Chemistry

**chemistchemicalApplied Chemistry**

Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems.

### Hund's rule of maximum multiplicity

**Hund's ruleHund's first rulerule of maximum multiplicity**

For multielectron atoms we must introduce some rules as that the electrons fill orbitals in a way to minimize the energy of the atom, in order of increasing energy, the Pauli exclusion principle, Hund's rule, and the aufbau principle.

The rule, discovered by Friedrich Hund in 1925, is of important use in atomic chemistry, spectroscopy, and quantum chemistry, and is often abbreviated to Hund's rule, ignoring Hund's other two rules.

### Dihydrogen cation

**hydrogen molecular ionH 2 + hydrogen molecule-ion**

An exact solution for the Schrödinger equation can only be obtained for the hydrogen atom (though exact solutions for the bound state energies of the hydrogen molecular ion have been identified in terms of the generalized Lambert W function).

Consequently, it is included as an example in most quantum chemistry textbooks.

### Electronic density

**electron densitydensityelectronic densities**

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.

In quantum mechanics, and in particular quantum chemistry, the electronic density is a measure of the probability of an electron occupying an infinitesimal element of space surrounding any given point.

### Møller–Plesset perturbation theory

**MP2Møller–Plessetab initio'' calculations**

This computational affordability and often comparable accuracy to MP2 and CCSD(T) (post-Hartree–Fock methods) has made it one of the most popular methods in computational chemistry.

Møller–Plesset perturbation theory (MP) is one of several quantum chemistry post-Hartree–Fock ab initio methods in the field of computational chemistry.

### Molecular Hamiltonian

**electronic molecular HamiltonianHamiltonianelectronic Hamiltonian**

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.

In atomic, molecular, and optical physics and quantum chemistry, the molecular Hamiltonian is the Hamiltonian operator representing the energy of the electrons and nuclei in a molecule.

### Coupled cluster

**CCSD(T)Coupled cluster theoryCC2**

This computational affordability and often comparable accuracy to MP2 and CCSD(T) (post-Hartree–Fock methods) has made it one of the most popular methods in computational chemistry.

Its most common use is as one of several post-Hartree–Fock ab initio quantum chemistry methods in the field of computational chemistry, but it is also used in nuclear physics.

### Kohn–Sham equations

**Kohn-Sham equationsKohn–ShamKohn–Sham method**

Modern day DFT uses the Kohn–Sham method, where the density functional is split into four terms; the Kohn–Sham kinetic energy, an external potential, exchange and correlation energies.

In physics and quantum chemistry, specifically density functional theory, the Kohn–Sham equation is the one electron Schrödinger equation (more clearly, Schrödinger-like equation) of a fictitious system (the "Kohn–Sham system") of non-interacting particles (typically electrons) that generate the same density as any given system of interacting particles.

### Molecular orbital

**molecular orbitalsorbitalorbitals**

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.

This qualitative approach to molecular orbital theory is part of the start of modern quantum chemistry.

### Diabatic

Their formula allows the transition probability between two diabatic potential curves in the neighborhood of an avoided crossing to be calculated.

In quantum chemistry, the potential energy surfaces are obtained within the adiabatic or Born–Oppenheimer approximation.

### Avoided crossing

**anticrossingnon-crossing ruleavoided crossing principle**

Their formula allows the transition probability between two diabatic potential curves in the neighborhood of an avoided crossing to be calculated.

In quantum physics and quantum chemistry, an avoided crossing (sometimes called intended crossing, non-crossing or anticrossing) is the phenomenon where two eigenvalues of an Hermitian matrix representing a quantum observable and depending on N continuous real parameters cannot become equal in value ("cross") except on a manifold of N-2 dimensions.

### Chemical bond

**bondbondschemical bonds**

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.

Most quantitative calculations in modern quantum chemistry use either valence bond or molecular orbital theory as a starting point, although a third approach, density functional theory, has become increasingly popular in recent years.

### Molecular dynamics

**dynamicsMDmolecular dynamic**

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).

This is a crude approximation because hydrogen bonds have a partially quantum mechanical and chemical nature.

### List of quantum chemistry and solid-state physics software

**Quantum chemistry computer programsList of quantum chemistry and solid state physics softwareQuantum chemistry and solid state physics software**

Quantum chemistry computer programs are used in computational chemistry to implement the methods of quantum chemistry.

### Electron localization function

**electron localization**

In quantum chemistry, the electron localization function (ELF) is a measure of the likelihood of finding an electron in the neighborhood space of a reference electron located at a given point and with the same spin.

### International Academy of Quantum Molecular Science

**IAQMSSo Hirata**

The members are "chosen among the scientists of all countries who have distinguished themselves by the value of their scientific work, their role of pioneer or leader of a school in the broad field of quantum chemistry, i.e. the application of quantum mechanics to the study of molecules and macromolecules".

### Friedrich Hund

**Hund**

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's cases, which are particular regimes in diatomic molecular angular momentum coupling, and Hund's rules, which govern atomic electron configurations, are important in spectroscopy and quantum chemistry.

### Physical chemistry

**physical chemistphysicochemicalphysical**

Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mechanics, analytical dynamics and chemical equilibrium.