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Chemical bonding computation method

The semi-empirical methods of HyperChem are quantum mechanical methods that can describe the breaking and formation of chemical bonds, as well as provide information about the distribution of electrons in the system. HyperChem s molecular mechanics techniques, on the other hand, do not explicitly treat the electrons, but instead describe the energetics only as interactions among the nuclei. Since these approximations result in substantial computational savings, the molecular mechanics methods can be applied to much larger systems than the quantum mechanical methods. There are many molecular properties, however, which are not accurately described by these methods. For instance, molecular bonds are neither formed nor broken during HyperChem s molecular mechanics computations the set of fixed bonds is provided as input to the computation. [Pg.159]

Extensive quantum chemical calculations have been reported for sulfur-rich compounds in the past two decades. These calculations were used to investigate molecular structures and spectroscopic properties, as well as to understand the nature chemical bonding and reaction mechanism. Many high-level ab initio calculations were used for interpretation of experimental data and for providing accurate predictions of molecular structures and thermochemical data where no reliable experimental values are available. In recent years, density functional calculations have been extensively tested and used on many first- and second-row compounds. These proven DFT methods look promising for larger systems because for their computational efficiency. [Pg.26]

In the end, mass spectrometry and ion techniques will continue to be powerful tools for the investigation of the structure, bonding, energetics, and reactivity of unusual organic molecules. New sophisticated techniques will continue to be developed and applied to interesting problems in physical organic chemistry. These studies, along with the continued improvements in computational methods (Chapter 9), provide means to obtain very detailed and accurate descriptions of chemical reactions. [Pg.239]

Unlike electrostatic forces, chemical forces between the probing tip and the probed surface have been shown to profoundly affect the tunneling current from a certain onset. Owing to the advent of first-principle methods and powerful computers, it could finally be resolved by a calculation of the combined tip-sample system [ 15 ]. The point of onset for chemical bonding on metals was found to be at a distance of 4—5 A. As the tip approaches the surface, chemical forces rapidly become large enough to... [Pg.98]

During the last few years, both neutral and cationic 1,3,2-diazaphospholes and NHP have been studied extensively by computational methods. The best part of these studies focused on a discussion of n-electron delocalization and their implication on chemical reactivities and stabilities, the explanation of the unique ionic polarization of exocyclic P-X bonds noted for some species, and the evaluation of structural and spectroscopic properties with the aim of helping in the interpretation of experimental data. [Pg.82]

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. [Pg.154]

Further theoretical studies supported by in situ spectroscopy and high-resolution microscopy are needed to be able to understand this unusually strong bonding between Cu and Ce. To apply such first-principles quantum chemical MD approach, new computational methods accelerating computational time by several orders of magnitude must be developed. [Pg.224]

Under the Born-Oppenheimer approximation, two major methods exist to determine the electronic structure of molecules The valence bond (VB) and the molecular orbital (MO) methods (Atkins, 1986). In the valence bond method, the chemical bond is assumed to be an electron pair at the onset. Thus, bonds are viewed to be distinct atom-atom interactions, and upon dissociation molecules always lead to neutral species. In contrast, in the MO method the individual electrons are assumed to occupy an orbital that spreads the entire nuclear framework, and upon dissociation, neutral and ionic species form with equal probabilities. Consequently, the charge correlation, or the avoidance of one electron by others based on electrostatic repulsion, is overestimated by the VB method and is underestimated by the MO method (Atkins, 1986). The MO method turned out to be easier to apply to complex systems, and with the advent of computers it became a powerful computational tool in chemistry. Consequently, we shall concentrate on the MO method for the remainder of this section. [Pg.106]

Quantum Systems in Chemistry and Physics is a broad area of science in which scientists of different extractions and aims jointly place special emphasis on quantum theory. Several topics were presented in the sessions of the symposia, namely 1 Density matrices and density functionals 2 Electron correlation effects (many-body methods and configuration interactions) 3 Relativistic formulations 4 Valence theory (chemical bonds and bond breaking) 5 Nuclear motion (vibronic effects and flexible molecules) 6 Response theory (properties and spectra atoms and molecules in strong electric and magnetic fields) 7 Condensed matter (crystals, clusters, surfaces and interfaces) 8 Reactive collisions and chemical reactions, and 9 Computational chemistry and physics. [Pg.434]

Karpins, M. and R. N. Porter. 1970. Atoms and Molecules. Menlo Park, CA W. A. Benjamin. Pauling, L. 1960. The Nature of the Chemical Bond. Ithaca, NY Cornell University Press. Westmoreland, P. R. and A. Z. Panagiotoponlos. 2004. Molecnlar-modeling methods and nse for product and process design. In Sixth International Conference on Foundations of Computer-aided Process Design, Flondas C. A. and R. Agrawal eds pp. 83-95. [Pg.147]

This book is about electronic charge distributions, chemical bonds, bond energy additivity in organic molecules, and the description of their relevant thermochemical properties, such as the energy of atomization, the enthalpy of formation, and the like, using computer-friendly methods. [Pg.3]


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