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Semiempirical
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 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
PM3 (chemistry)
PM3, or Parametric Method 3, 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. The PM3 method uses the same formalism and equations as the AM1 method. The only differences are: 1) PM3 uses two Gaussian functions for the core repulsion function, instead of the variable number used by AM1 (which uses between one and four Gaussians per element); 2) the numerical values of the parameters are different. The other differences lie in the philosophy and methodology used during the parameterization: whereas AM1 takes some of the parameter values from spectroscopical measurements, PM3 treats them as optimizable values. The method was developed by J. J. P. Stewart and first published in 1989. It is implemented in the MOPAC program (of which the older versions are public domain), along with the related RM1, AM1, MNDO and MINDO methods, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spartan (chemistry Software)
Spartan is a molecular modelling and computational chemistry application from Wavefunction. It contains code for molecular mechanics, semi-empirical methods, ''ab initio'' models, density functional models, post-Hartree–Fock models, and thermochemical recipes including G3(MP2) and T1. Quantum chemistry calculations in Spartan are powered by Q-Chem. Primary functions are to supply information about structures, relative stabilities and other properties of isolated molecules. Molecular mechanics calculations on complex molecules are common in the chemical community. Quantum chemical calculations, including Hartree–Fock method molecular orbital calculations, but especially calculations that include electronic correlation, are more time-consuming in comparison. Quantum chemical calculations are also called upon to furnish information about mechanisms and product distributions of chemical reactions, either directly by calculations on transition states, or based on Hammond's p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SAM1
SAM1, or "Semiempirical ab initio Model 1", is a semiempirical quantum chemistry method for computing molecular properties. It is an implementation the general Neglect of Differential Diatomic Overlap (NDDO) integral approximation, and is efficient and accurate. Related methods are AM1, PM3 and the older MNDO. SAM1 was developed by M.J.S. Dewar and co-workers at the University of Texas and the University of Florida. Papers describing the implementation of the method and its results were published in 1993 and 1994. The method is implemented in the AMPAC program produced bSemichem SAM1 builds on the success of the Dewar-style semiempirical models by adding two new aspects to the AM1/PM3 formalism: #Two-electron repulsion integrals (TERIs) are computed from a minimal basis set of contracted Gaussian functions, as opposed to the previously used multipole expansion. Note that the NDDO approximation is still in effect, and that only a few of the possible TERIs are explicitly compu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
AMPAC
AMPAC is a general-purpose semiempirical quantum chemistry program. It is marketed by Semichem, Inc. and was developed originally by Michael Dewar and his group. The first version of AMPAC (2.1) was made available in 1985 through the Quantum Chemistry Program ExchangeQCPE. Subsequent versions were released through the same source, representing minor updates and optimized versions for other platforms. In 1992Semichem, Inc.was formed at Professor Dewar's urging to maintain and market the program. ''AMPAC 4.0 with Graphical User Interface'' was released in August of that year. Semichem's current version of AMPAC is 10. AMPAC current implements the SAM1, AM1, MNDO, MNDO/d, PM3, MNDOC MINDO/3, RM1 and PM6 semi-empirical methods and AMSOL and COSMO salvation models. Sethis pagefor a detailed description of AMPAC's current capabilities. See also * Quantum chemistry computer programs Quantum chemistry computer programs are used in computational chemistry to implement the me ... [...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] |
Pariser–Parr–Pople Method
In molecular physics, the Pariser–Parr–Pople method applies Computational Chemistry#Semiempirical methods, 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 Chemical structure, structure and reactivity (chemistry), reactivity of a molecule ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Roald Hoffmann
Roald Hoffmann (born Roald Safran; July 18, 1937) is a Polish Americans, Polish-American theoretical chemistry, theoretical chemist who won the 1981 Nobel Prize in Chemistry. He has also published plays and poetry. He is the Frank H. T. Rhodes Professor of Humane Letters, Emeritus, at Cornell University, in Ithaca, New York. Early life Escape from the Holocaust Hoffmann was born in Złoczów, Second Polish Republic (now Zolochiv, Ukraine), to a Polish-Jewish family, and was named in honor of the Norway, Norwegian explorer Roald Amundsen. His parents were Clara (Rosen), a teacher, and Hillel Safran, a civil engineer. After Germany invaded Poland and occupied the town, his family was placed in a labor camp where his father, who was familiar with much of the local infrastructure, was a valued prisoner. As the situation grew more dangerous, with prisoners being transferred to extermination camps, the family bribed guards to allow an escape. They arranged with a Ukrainian neighbor ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Extended Hückel Method
The extended Hückel method is a semiempirical quantum chemistry method, developed by Roald Hoffmann since 1963. It is based on the Hückel method but, while the original Hückel method only considers pi orbitals, the extended method also includes the sigma orbitals. The extended Hückel method can be used for determining the molecular orbitals, but it is not very successful in determining the structural geometry of an organic molecule. It can however determine the relative energy of different geometrical configurations. It involves calculations of the electronic interactions in a rather simple way for which the electron-electron repulsions are not explicitly included and the total energy is just a sum of terms for each electron in the molecule. The off-diagonal Hamiltonian matrix elements are given by an approximation due to Wolfsberg and Helmholz that relates them to the diagonal elements and the overlap matrix element. H_ = KS_ \dfrac ''K'' is the Wolfsberg–Helmholz c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
NDDO
In computational chemistry, NDDO (neglect of diatomic differential overlap) is a formalism that was first introduced by John Pople and it is now the basis of most successful semiempirical methods. While INDO added all one-centre two electron integrals to the CNDO/2 formalism, NDDO adds all two centre integrals for repulsion between a charge distribution on one centre and a charge distribution on another centre.J. Pople and D. Beveridge, ''Approximate Molecular Orbital Theory'', McGraw-Hill, 1970 Otherwise the zero-differential overlap approximation is used. A common software program is MOPAC (Molecular Orbital PACkage). In the Neglect of Diatomic Differential Overlap (NDDO) method the overlap matrix S is replaced by the unit matrix. This allows one to replace the Hartree–Fock secular equation , H–ES, = 0 with a simpler equation , H–E, =0. The two-electron integrals from the NDDO approximation can either be one-, two-, three- or four-centered. The one-and two-centered integ ... [...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] |
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] |
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] |
