Quantum chemistry

AM1, or Austin Model 1, is a semi-empirical method for the quantum calculation of molecular electronic structure in computational chemistry. It is based on the Neglect of Differential Diatomic Overlap integral approximation. Specifically, it is a generalization of the modified neglect of differential diatomic overlap approximation. Related methods are PM3 and the older MINDO. ...more on Wikipedia about "AM1"

Autoionization is a process by which atoms or molecules spontaneously transition from an electrically neutral state to a lower-energy ionized state. ...more on Wikipedia about "Autoionization"

(Avoided crossing) The eigenvalues of a Hermitian matrix depending on N continuous real parameters cannot cross except at a manifold of N-1 dimensions. In the case of a diatomic molecule, this means that the eigenvalues do not cross. In the case of a triatomic molecule, this means that the eignevalues can intersect on a line only (see conical intersection). ...more on Wikipedia about "Avoided crossing"

In modern computational chemistry, quantum chemical calculations are typically performed within a finite set of basis functions. In these cases, the wavefunctions under consideration are all represented as vectors, the components of which correspond to coefficients in a linear combination of the basis functions in the basis set used. The operators are then represented as matrices, (rank two tensors), in this finite basis. In this article, basis function and atomic orbital are sometimes used interchangeably, although it should be noted that these basis functions are usually not actually the exact atomic orbitals, even for the corresponding hydrogen-like atoms, due to approximations and simplifications of their analytic formulas. ...more on Wikipedia about "Basis set (chemistry)"

The Born- Oppenheimer approximation, also known as the adiabatic approximation, is a technique used in quantum chemistry and condensed matter physics in order to de-couple the motion of nuclei and electrons (i.e. to separate the variables corresponding to the nuclear and electronic coordinates in the Schrödinger equation associated to the molecular Hamiltonian). It is based upon the fact that typical electronic velocities far exceed those of nuclei. ...more on Wikipedia about "Born-Oppenheimer approximation"

In quantum chemistry, a Complete Active Space is a type of classification of molecular orbitals. Spatial orbitals are classified as belonging to three classes: ...more on Wikipedia about "Complete active space"

Configuration interaction (CI) is a post Hartree-Fock linear variational method for solving the nonrelativistic Schrödinger equation within the Born-Oppenheimer approximation for a quantum chemical multi-electron system. Two meanings are connected to the term configuration interaction in this context. Mathematically, configuration simply describes the linear combination of Slater determinants used for the wave function. In terms of a specification of orbital occupation (for instance, (1s)²(2s)²(2p)¹...), interaction means the mixing (interaction) of different electronic configurations (states). Due to the long CPU time and immense hardware required for CI calculations, the method is limited to relatively small systems. ...more on Wikipedia about "Configuration interaction"

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In quantum chemistry, a configuration state function, or CSF, is a Slater determinant, representing an approximate electronic wavefunction within a molecule. The CSF can represent either an approximate ground state electronic configuration, or an excited state electronic configuration. In post-Hartree-Fock methods, CSFs are often used with coefficients to form linear combinations of CSFs, in order to approximate the effect of electron correlation within the molecule. ...more on Wikipedia about "Configuration state function"

In quantum chemistry, a conical intersection of two potential energy surfaces of the same spatial and spin symmetries is the set of molecular geometry points where the two potential energy surfaces are degenerate (intersect). ...more on Wikipedia about "Conical intersection"

Coupled cluster (CC) method is a technique used for description of the many-body systems. The method was initially developed by Fritz Coester and Hermann Kümmel in 1950s for studying nuclear physics phenomena but it became more frequently used after Jiři Čížek and Josef Paldus reformulated the method for studying electronic correlation in atoms and molecules in 1960s. It is now one of the most prevalent methods in quantum chemistry that include electronic correlation. It is important to note that this method is applicable specifically to fermionic systems; in the case of computational chemistry, it is specifically applicable to electron systems, as specified above. ...more on Wikipedia about "Coupled cluster"

Density functional theory (DFT) is one of the most popular approaches to quantum mechanical many-body electronic structure calculations of molecular and condensed matter systems. ...more on Wikipedia about "Density functional theory"

In quantum chemistry, the potential energy surfaces are obtained within the adiabatic or Born-Oppenheimer approximation. This corresponds to a representation of the molecular wave function where the variables corresponding to the molecular geometry and the electronic degrees of freedom are separated. The non separable terms are due to the nuclear kinetic energy terms in the molecular Hamiltonian and are said to couple the potential energy surfaces. In the neighbourhood of an avoided crossing or conical intersection, these terms cannot be neglected. One therefore usually perform one unitary transformation from the adiabatic representation to the so-called diabatic representation in which the nuclear kinetic energy operator is diagonal. In this representation, the coupling is due to the electronic energy and is a scalar quantity which is much more easy to estimate numerically. ...more on Wikipedia about "Diabatic"

Direct quantum chemistry covers a set of quantum chemical methods not using the Born-Oppenheimer representation. Direct quantum chemistry considers the motion of the nuclei and the electrons on the same time scales. The method therefore considers the molecular Hamiltonian as a whole without trying to solve separately the Schrödinger equation associated to the electronic molecular Hamiltonian. Though the method is non adiabatic method it is distinguishable from most non adiabatic methods for treating the molecular dynamics, which typically use the Born-Oppenheimer representation, but become nonadiabatic by considering vibronic coupling explicitly. ...more on Wikipedia about "Direct quantum chemistry"

Electron cloud is a term used for introducing the concept of wavefunction in low-level pedagogical introductions to atomic physics, molecular physics, chemistry or quantum chemistry. This idea corresponds to delocated electrons moving or standing like clouds around the atomic or molecular nuclei. This is indeed a better image than the very common image provided by the Bohr model which commonly leads to a visualisation of electrons driving around the nuclei along orbits like the planets around the sun. ...more on Wikipedia about "Electron cloud" I wish I had a shortopedia. Quantum_chemistry

In atomic physics, the electron configuration is the arrangement of electrons in an atom, molecule or other body. Specifically, it is the placement of electrons into atomic, molecular, or other forms of electron orbitals. ...more on Wikipedia about "Electron configuration"

In solid state physics, an electron hole (usually referred to simply as a hole) is the absence of an electron from the otherwise full valence band. A full (or nearly full) valence band is present in semiconductors and insulators. The concept of a hole is essentially a simple way to analyse the electronic transitions within the valence band. ...more on Wikipedia about "Electron hole"

In atomic physics, an electron shell, also know as a main energy level, is a group of atomic orbitals with the same value of the principal quantum number n. Electron shells are made up of one or more electron subshells, or sublevels, which have two or more orbitals with the same angular momentum quantum number l. Electron shells make up the electron configuration of an atom. It can be shown that the number of electrons that can reside in a shell is equal to $2n^2$ . ...more on Wikipedia about "Electron shell"

Electronic correlation refers to the interaction between electrons in a quantum system whose electronic structure is being considered. The term correlation stems from mathematical statistics and means that two distribution functions, f(x) and g(y), are not independent of each other. ...more on Wikipedia about "Electronic correlation"

In quantum mechanics, and in particular in quantum chemistry, the electronic density $\rho$ corresponding to an N-electron wavefunction $\Psi^{(N)}$ is the one-electron function given by ...more on Wikipedia about "Electronic density"

The electronic molecular Hamiltonian is the term of the molecular Hamiltonian obtained when the molecular geometry is frozen. Within the Born-Oppenheimer approximation, the electronic Hamiltonian is said to depend adiabatically on the molecular geometry. Its discrete eigenvalues are called potential energy surfaces and the corresponding eigenstates the electronic states of the molecule. The electronic states are labelled according to their group representation and spin multiplicity. ...more on Wikipedia about "Electronic molecular Hamiltonian"

Energy levels are said to be degenerate, if the same energy level is obtained by more than one quantum mechanical state. They are then called degenerate energy levels. ...more on Wikipedia about "Energy level" Everybody should like www.shortopedia.com

In quantum mechanics, a Feshbach resonance, in contrast to a shape resonance, is a resonance of a system with more than one degree of freedom, which would turn into a bound state if the coupling between some degrees of freedom and the degrees of freedom associated to the fragmentation ( reaction coordinates) were set to zero. ...more on Wikipedia about "Feshbach resonance"

In quantum mechanics, the Fock matrix is a matrix approximating the single-electron energy operator of a given quantum system in a given set of basis vectors. ...more on Wikipedia about "Fock matrix"

A forbidden mechanism is a commonly misunderstood concept in chemistry. It usually refers to electrons changing state (e.g. a forbidden transition) but it can refer to some reactions too. The common misconception is in the term "forbidden" itself and its usual English connotation. In chemistry, forbidden means absolutely impossible due to natural laws but with the assumption of an ideal symmetry. Since that symmetry condition is not always met, forbidden transitions usually mean a very, very slow mechanism. A good example is phosphorescence where the spin state transition is "forbidden". This is why phosphorescent material glows for so long compared to fluorescence which involves a very short non-forbidden transition. ...more on Wikipedia about "Forbidden mechanism"

The Franck- Condon principle is a rule in quantum chemistry that allows one to predict the intensity of a vibronic transition. The principle states that a transition from one energy level to another will be more likely to happen if the wave functions of the two energy levels overlap. The more the energy levels overlap, the more likely the transition. Because wave functions are closely related to the shape of a molecule, an equivalent formulation of the Franck-Condon principle is that that the structure of a molecule does not change during certain fast reactions. The implication is that for such processes orbital symmetry must be conserved, otherwise they are forbidden. ...more on Wikipedia about "Franck-Condon principle" Connect with http://www.shortopedia.com.

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