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Zero-differential Overlap
Zero differential overlap is an approximation in computational molecular orbital theory that is the central technique of semi-empirical methods in quantum chemistry. When computers were first used to calculate bonding in molecules, it was only possible to calculate diatomic molecules. As computers advanced, it became possible to study larger molecules, but the use of this approximation has always allowed the study of even larger molecules. Currently semi-empirical methods can be applied to molecules as large as whole proteins. The approximation involves ignoring certain integrals, usually two-electron repulsion integrals. If the number of orbitals used in the calculation is N, the number of two-electron repulsion integrals scales as N4. After the approximation is applied the number of such integrals scales as N2, a much smaller number, simplifying the calculation. Details of approximation If the molecular orbitals \mathbf_i \ are expanded in terms of ''N'' basis functions, \mathbf ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Molecular Orbital
In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The terms ''atomic orbital'' and ''molecular orbital'' were introduced by Robert S. Mulliken in 1932 to mean ''one-electron orbital wave functions''. At an elementary level, they are used to describe the ''region'' of space in which a function has a significant amplitude. In an isolated atom, the orbital electrons' location is determined by functions called atomic orbitals. When multiple atoms combine chemically into a molecule, the electrons' locations are determined by the molecule as a whole, so the atomic orbitals combine to form molecular orbitals. The electrons from the constituent atoms occupy the molecular orbitals. Mathematically, molecular orbitals are an approximate solution to the Schrödin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Semi-empirical Quantum Chemistry Methods
Semi-empirical quantum chemistry methods are based on the Hartree–Fock formalism, but make many approximations and obtain some parameters from empirical data. They are very important in computational chemistry for treating large molecules where the full Hartree–Fock method without the approximations is too expensive. The use of empirical parameters appears to allow some inclusion of electron correlation effects into the methods. Within the framework of Hartree–Fock calculations, some pieces of information (such as two-electron integrals) are sometimes approximated or completely omitted. In order to correct for this loss, semi-empirical methods are parametrized, that is their results are fitted by a set of parameters, normally in such a way as to produce results that best agree with experimental data, but sometimes to agree with ''ab initio'' results. Type of simplifications used Semi-empirical methods follow what are often called empirical methods where the two-electron p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quantum Chemistry
Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions to physical and chemical properties of Molecule, molecules, Material, materials, and solutions at the atomic level. These calculations include systematically applied approximations intended to make calculations computationally feasible while still capturing as much information about important contributions to the computed Wave function, wave functions as well as to observable properties such as structures, spectra, and thermodynamic properties. Quantum chemistry is also concerned with the computation of quantum effects on molecular dynamics and chemical kinetics. Chemists rely heavily on spectroscopy through which information regarding the Quantization (physics), quantization of energy on a molecular scale can be obtained. Common metho ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ab Initio Quantum Chemistry Methods
''Ab initio'' quantum chemistry methods are computational chemistry methods based on quantum chemistry. The term was first used in quantum chemistry by Robert Parr and coworkers, including David Craig in a semiempirical study on the excited states of benzene. The background is described by Parr. ''Ab initio'' means "from first principles" or "from the beginning", implying that the only inputs into an ''ab initio'' calculation are physical constants. ''Ab initio'' quantum chemistry methods attempt to solve the electronic Schrödinger equation given the positions of the nuclei and the number of electrons in order to yield useful information such as electron densities, energies and other properties of the system. The ability to run these calculations has enabled theoretical chemists to solve a range of problems and their importance is highlighted by the awarding of the Nobel prize to John Pople and Walter Kohn. Accuracy and scaling ''Ab initio'' electronic structure method ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Post-Hartree–Fock
In computational chemistry, post-Hartree–Fock methods are the set of methods developed to improve on the Hartree–Fock (HF), or self-consistent field (SCF) method. They add electron correlation which is a more accurate way of including the repulsions between electrons than in the Hartree–Fock method where repulsions are only averaged. Details In general, the SCF procedure makes several assumptions about the nature of the multi-body Schrödinger equation and its set of solutions: * For molecules, the Born–Oppenheimer approximation is inherently assumed. The true wavefunction should also be a function of the coordinates of each of the nuclei. * Typically, relativistic effects are completely neglected. The momentum operator is assumed to be completely nonrelativistic. * The basis set is composed of a finite number of orthogonal functions. The true wavefunction is a linear combination of functions from a complete (infinite) basis set. * The energy eigenfunctions are a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pariser–Parr–Pople Method
In molecular physics, the Pariser–Parr–Pople method applies semi-empirical quantum mechanical methods to the quantitative prediction of electronic structures and spectra, in molecules of interest in the field of organic chemistry. Previous methods existed—such as the Hückel method which led to Hückel's rule—but were limited in their scope, application and complexity, as is the Extended Hückel method. This approach was developed in the 1950s by Rudolph Pariser with Robert Parr and co-developed by John Pople. J. A. Pople, Transactions of the Faraday Society, 49, 1375, (1953) It is essentially a more efficient method of finding reasonable approximations of molecular orbitals, useful in predicting physical and chemical nature of the molecule under study since molecular orbital characteristics have implications with regards to both the basic structure and reactivity of a molecule. This method used the zero-differential overlap (ZDO) approximation to reduce the p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
CNDO/2
CNDO is the abbreviation for Complete Neglect of Differential Overlap, one of the first semi empirical methods in quantum chemistry. It uses two approximations: *core approximation - only the outer valence electrons are explicitly included. * zero-differential overlap CNDO/2 is the main version of CNDO. The method was first introduced by John Pople and coworkers. Background An earlier method was Extended Hückel method, which explicitly ignores electron-electron repulsion terms. It was a method for calculating the electronic energy and the molecular orbitals. CNDO/1 and CNDO/2 were developed from this method by explicitly including the electron-electron repulsion terms, but neglecting many of them, approximating some of them and fitting others to experimental data from spectroscopy. Methodology Quantum mechanics provides equations based on the Hartree–Fock method and the Roothaan equations that CNDO uses to model atoms and their locations. These equations are solved iterati ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
INDO
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{{disambig ...
Indo may refer to: * Indo-, a prefix indicating India or the Indian Subcontinent * Indonesia, a country in Asia ** INDO LINES, callsign of Indonesian Airlines ** Indo people, people of mixed European and Indonesian ancestry ** Indo cuisine, fusion cuisine of Indonesian and European * INDO, the Intermediate Neglect of Differential Overlap semi-empirical method * Indo (apple), an apple cultivar * Irish Independent, commonly nicknamed 'The Indo' * The slang term 'endo' or 'indo' is an AAVE prounuciation for "indoor"-grown marijuana. * Palacio de Indo, a former palace in Madrid See also * Endo (other) Endo may refer to: * Endo or stoppie, a motorcycle and bicycle trick in which the back wheel is lifted by abruptly applying the front brake * Endo people, an ethnic group in Kenya ** Endo language, the native language of the Endo people * Endo (ba ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
MINDO
MINDO, or Modified Intermediate Neglect of Differential Overlap is a semi-empirical method for the quantum calculation of molecular electronic structure in computational chemistry. It is based on the Intermediate Neglect of Differential Overlap (INDO) method of John Pople. It was developed by the group of Michael Dewar and was the original method in the MOPAC program. The method should actually be referred to as MINDO/3. It was later replaced by the MNDO MNDO, or Modified Neglect of Diatomic Overlap is a semi-empirical method for the quantum calculation of molecular electronic structure in computational chemistry. It is based on the Neglect of Diatomic Differential Overlap integral approximation. ... method, which in turn was replaced by the PM3 and AM1 methods. References * Semiempirical quantum chemistry methods {{quantum-chemistry-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
ZINDO
ZINDO is a semi-empirical quantum chemistry method used in computational chemistry. It is a development of the INDO method. It stands for Zerner's Intermediate Neglect of Differential Overlap, as it was developed by Michael Zerner and his coworkers in the 1970s. Unlike INDO, which was really restricted to organic molecules and those containing the atoms B to F, ZINDO covers a wide range of the periodic table, even including the rare-earth elements. There are two distinct versions of the method: * ZINDO/1 – for calculating ground-state properties such as bond lengths and bond angles. It refers to a SCF (RHF or ROHF) calculation with the INDO/1 level as suggested by Pople, which provides the reference state MO coefficients. Ground-state dipole moments and ionization potentials are in general very accurate. Geometry optimizations are erratic, what prompted Zerner's group to improve the performance of the code in the late 1990sJ. D. Da Motta Neto, M. Zerner, Int. J. Quantum Chem. 2 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SINDO
SINDO, is one of many semi-empirical quantum chemistry methods. It stands for symmetric orthogonalised INDO and was developed by K. Jug and coworkers. Like MINDO, it is a development of the INDO method. The main development is the inclusion of d orbitals for atoms of the second row of the periodic table. It performs better for hypervalent compounds than other semiempirical methods.K. Jug and J. Schulz, Journal of Computational Chemistry The ''Journal of Computational Chemistry'' is a peer-reviewed scientific journal published since 1980 by John Wiley & Sons. It covers research, contemporary developments in theory and methodology, and applications in all areas of computational chem ..., 9, 40, (1988) References Semiempirical quantum chemistry methods {{quantum-chemistry-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
