Quantum mechanics, semiempirical

M.S. Daw and M.I. Baskes, Semiempirical, quantum mechanical calculation of hydrogen... 

The last point has been studied more quantitatively for the electrolyte LiOjCCH Fy (x+y=3) / DMSO [97,105], Semiempirical quantum-mechanical calculations with the help of MOP AC [143] show that the mean electron density at the oxygen atoms q(0) decreases for these acetates by about 0.1 unit with increasing fluorine content of the anion [97]. As a consequence ... 

The complete theory of catalysis, which would start with the isolated reaction participants, was not available until now because of the lack of adequate knowledge of the participants themselves (even the complete theory of the isolated participants, starting from the first principles, is still lacking). However, in analogy with the homogeneous chemical reactions one can expect that the quantum chemical approach, based on the semiempirical quantum mechanical methods, could be a prospective one. 

SemiChem products are available at http //www.semichem.com/prods.html. AMPAC , available as a stand-alone product with Windows-based and work-station-level interfaces, is a semiempirical quantum mechanical program featuring SAMI, AMI, MNDO, MNDO/d, PM3, MNDO/C, and MINDO/3 semiempirical... 

MOPAC is a general-purpose semiempirical molecular orbital program for the study of chemical structures and reactions. It is available in desktop PC running Windows, Macintosh OS, and Unix-based workstation versions. It uses semiempirical quantum mechanical methods that are based on Hartree-Fock (HF) theory with some parameterized functions and empirically determined parameters replacing some sections of the complete HF treatment. The approximations in... 

Giesen, D. J., Storer, J., Cramer, C. J. and Truhlar, D. J. General semiempirical quantum mechanical solvation model for nonpolar solvation free energies, n-hexadacane., J.Am. Chem.Soc., 117 (1995), 1057-1068... 

In a very extensive test of the SM5 method (a type of GBM), Hawkins et al. found the average absolute deviation in AGsoivation to be 0.38 kcal/mole for 260 molecular solutes in water and in 90 organic solvents 131 for ions in water, it was 3.8 kcal/mole, for experimental AGsolvation between -58 and -110 kcal/mole. Semiempirical quantum-mechanical procedures were used. 

Hypercube, Inc. at http //www.hyper.com offers molecular modeling packages under the HyperChem name. HyperChem s newest version, Hyper-Chem Release 7.5, is a full 32-bit application, developed for the Windows 95, 98, NT, ME, 2000, and XP operating systems. Density Functional Theory (DFT) has been added as a basic computational engine to complement Molecular Mechanics, Semiempirical Quantum Mechanics and ab initio Quantum Mechanics. The DFT engine includes four combination or hybrid functions, such as the popular B3-LYP or Becke-97 methods. The Bio+ force field in HyperChem represents a version of the Chemistry at HARvard using Molecular Mechanics (CHARMM) force field. Release 7.5 of HyperChem updates... 

At present two parallel approaches are being undertaken to determine the PES or the Cj The ab initio and semiempirical quantum mechanical methods. Ab initio methods attempt to undertake the just-mentioned calculations rigorously without relying on the use of any information other than the basis sets resulting accuracy of the calculations large basis sets... 

The most important uncertainty associated with the determination of is related to our ability to predict heats of formation of species. At present, AH( values for stable species can be predicted within 5kcal/mol using various forms of additivity principles, provided these rules are applicable. Estimations based on semiempirical quantum mechanics are more general and can be as accurate. Although ab initio calculations can be more accurate, they are computationally prohibitive. For radical species, the associated uncertainties in AHf generally are larger. 

Small monosaccharides have molecular sizes at the upper limit of the range that is currently treatable with initio methods. An exaiqple of the application of initio calculations to carbohydrates is given in the paper by Garrett and Serianni in this volume. Semiempirical quantum mechanical calculations, which use simplified molecular Hamiltonians with parameters taken from experiment, extend quantum mechanical calculations to larger molecules. However, the reliability is reduced compared to the best ab initio results. 

There is a 3 kcal difference in energy between the energy difference shown in Fig. 38 and the McGrath results. The calculations used for Fig. 38 are based on a lower level, semiempirical quantum mechanics calculation, while the McGarth calculations utilize computationally more expensive ab initio methods. 

A number of papers have appeared recently in which semiempirical quantum mechanical methods, such as the complete neglect of differential overlap (CNDO), incomplete neglect of differential overlap (INDO), or Hiickel methods, have been applied to electron-deficient systems in an attempt to calculate their properties (31, 49, 64, 75, 77, 78, 89, 90, 92). Although the quantitative results of these calculations must be treated with great care, they do provide an indication of some of the parameters that determine formation and stability of electron-deficient bonded systems. 

It is possible to approach the problem of annular tautomerism using semiempirical quantum-mechanical calculations (Section V,B) the results are in accord with the empirical rules used in this section. 

As part of a theoretical examination of the factors controlling the catalytic efficiency of a transmethylation enzyme (catechol (9-mcthyltransferase), the reaction mechanism of the non-enzymic transmethylation of catechol by, -adcnosylmethionine (AdoMet, as modelled by sulfonium ion) has been elucidated by using ab initio and semiempirical quantum mechanical methods.97 The gas-phase reaction between catecholate and sulfonium is extremely fast, involving no overall barrier, and the reaction profile to some extent resembles that of a typical gas-phase, S N 2 reaction. However, in aqueous solution, this reaction is very slow, with a predicted barrier of 37.3 kealmol-1. Good agreement between calculated KIEs for the model reaction and measured KIEs for the enzymic reaction suggests that the transition states are similar. 

We have performed a series of semiempirical quantum-mechanical calculations of the molecular hyperpolarzabilities using two different schemes the finite-field (FF), and the sum-over-state (SOS) methods. Under the FF method, the molecular ground state dipole moment fJ.g is calculated in the presence of a static electric field E. The tensor components of the molecular polarizability a and hyperpolarizability / are subsequently calculated by taking the appropriate first and second (finite-difference) derivatives of the ground state dipole moment with respect to the static field and using... 

D. Arad, R. Langridge and P. Kollman, A simulation of the sulfur attack in catalytic pathway of papain using molecular mechanics and semiempirical quantum mechanics, J. Am. Chem. Soc., 112 (1990) 491-502. 

The topic of interactions between Lewis acids and bases could benefit from systematic ab initio quantum chemical calculations of gas phase (two molecule) studies, for which there is a substantial body of experimental data available for comparison. Similar computations could be carried out in the presence of a dielectric medium. In addition, assemblages of molecules, for example a test acid in the presence of many solvent molecules, could be carried out with semiempirical quantum mechanics using, for example, a commercial package. This type of neutral molecule interaction study could then be enlarged in scope to determine the effects of ion-molecule interactions by way of quantum mechanical computations in a dielectric medium in solutions of low ionic strength. This approach could bring considerable order and a more convincing picture of Lewis acid base theory than the mixed spectroscopic (molecular) parameters in interactive media and the purely macroscopic (thermodynamic and kinetic) parameters in different and varied media or perturbation theory applied to the semiempirical molecular orbital or valence bond approach [11 and references therein]. 

The semiempirical quantum mechanical approximation82-86 that has been called CNDO/2 (for complete neglect of differential overlap) was used to calculate binding energies in the trigonal bipyramidal configuration and in a series of model situations that are pertinent to the TR and the BPR mechanisms in a variety of molecules.81 This approximation to the solution of the LCAO-SCF equations yields results that provide, at least, a trend in the real systems involved. It has been shown that, with reasonable approximations and parametrization, this method offers an adequate compromise with respect to the more rigorous and accurate but far more laborious and expensive ah initio methods.82-85... 

The electron affinity of 2- and 5-nitroimidazoles calculated by a semiempirical quantum-mechanical method satisfactorily correlates with experimental values of one-electron reduction potentials measured by pulse radiolysis [942],... 

Barriers to inversion of about 200 kJ mol have been determined for tertiary arsines, both by experimental and semiempirical quantum mechanical calculations see Semi Empirical Theoretical Methods) The inversion barriers in arsines are considered to be the maximum for the group 15 elements, with values being about 60 kJ mol higher than for corresponding phosphines. For partially and fully substituted silylarsines, the inversion barrier at arsenic decreases as more silicon atoms are directly attached. 

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