Quantum mechanical computations

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... 

The first to consider quantum mechanical computation was Benioff ([beni81], [beni82a], [beni82b] and [beni84]). His point of departure was to suppose that since... 

A more elegant model of quantum mechanical computers was introduced by Feynman ([feyii85], [feynSb]). Although the formalism is similar to Benioff s - both approaches seek to define an appropriate quantum mechanical Hamiltonian H whose time evolution effectively represents the execution of a desired computation... 

The electronic structure and physical properties of any molecule can in principle be determined by quantum-mechanical calculations. However, only in the last 20 years, with the availability and aid of computers, has it become possible to solve the necessary equations without recourse to rough approximations and dubious simplifications2. Computational chemistry is now an established part of the chemist s armoury. It can be used as an analytical tool in the same sense that an NMR spectrometer or X-ray diffractometer can be used to rationalize the structure of a known molecule. Its true place, however, is a predictive one. Therefore, it is of special interest to predict molecular structures and physical properties and compare these values with experimentally obtained data. Moreover, quantum-mechanical computations are a very powerful tool in order to elucidate and understand intrinsic bond properties of individual species. 

The site http //www.nyu.edu/pages/mathmol/modules/water/info water.html hosts a nice discussion of water, including two short video clips (1) the quantum-mechanically computed movement of two water molecules united by means of a single hydrogen bond, at http //www.nyu.edu/pages/mathmol/modules/water/dimer.mpg-, (2) a short film of several hundred water molecules dancing within a cube at http //www. nyu. edu/pages/mathmol/modules/water/water dynamics.mpg. 

The two avenues above recalled, namely ab-initio computations on clusters and Molecular Dynamics on one hand and continuum model on the other, are somewhat bridged by those techniques where the solvent is included in the hamiltonian at the electrostatic level with a discrete representation [13,17], It is important to stress that quantum-mechanical computations imply a temperature of zero K, whereas Molecular Dynamics computations do include temperature. As it is well known, this inclusion is of paramount importance and allows also the consideration of entropic effects and thus free-energy, essential parameters in any reaction. 

For DFT and ab initio quantum mechanical computations of the cydopropylidene ringopening process, the decisive step of this allene synthesis, see H. F. Bettinger,... 

Drexel undergraduate students in both the lecture and the laboratory of physical chemistry have b n using TKISolver for such calculations as least squares fitting of experimental data, van der Waals gas calculations, and quantum mechanical computations (plotting particle-in-a-box wavefunctions, atomic orbital electron densities, etc.). I use TKISolver in lectures (on a Macintosh with video output to a 25" monitor) to solve simple equations and plot functions of chemical interest. 

There is no general-purpose or user-friendly software available for statistical mechanical computations, in comparison with the availability of useful and convenient tools to do single molecule quantum mechanical computations. There are many properties that can be computed, but users usually have to write their own computer programs to do each computation, so that it is a small research project instead of a convenient tool for product engineers. Westmoreland and Panagiotopoulos (2004) said that the availability... 

Quantum mechanical computation of molecular structure HyperChem, molecular structure Spartan Gaussian Cache. 

This semiclassical turnover theory differs significantly from the semiclassical turnover theory suggested by Mel nikov, who considered the motion along the system coordinate, and quantized the original bath modes and did not consider the bath of stable normal modes. In addition, Mel nikov considered only Ohmic friction. The turnover theory was tested by Topaler and Makri, who compared it to exact quantum mechanical computations for a double well potential. Remarkably, the results of the semiclassical turnover theory were in quantitative agreement with the quantum mechanical results. 

Recently there has emerged the beginning of a direct, operational link between quantum chemistry and statistical thermodynamic. The link is obtained by the ability to write E = V Vij—namely, to write the output of quantum-mechanical computations as the standard input for statistical computations, It seems very important that an operational link be found in order to connect the discrete description of matter (X-ray, nmr, quantum theory) with the continuous description of matter (boundary conditions, diffusion). The link, be it a transformation (probably not unitary) or other technique, should be such that the nonequilibrium concepts, the dissipative structure concepts, can be used not only as a language for everyday biologist, but also as a tool of quantitation value, with a direct, quantitative and operational link to the discrete description of matter. 

The full details of the quantum-mechanical computation to obtain the pair potential are available in O. Matsuoka, M. Yoshimine, and E. 

Thdry, V., Rinaldi, D., Rivail, J.-L., Maigret, B., and Ferenczy, G. G. 1994. Quantum Mechanical Computations on Very Large Molecular Systems The Local Self-consistent Field Method , J. Comput. Chem., 15, 269. 

The numerical evaluation of the energies of orbitals and states is fundamentally a matter of making quantum mechanical computations. As indicated in Chapter 1, quantum mechanics per se is not the subject of this book, and indeed we have tried in general to avoid any detailed treatment of methods for solving the wave equation, emphasis being placed on the properties that the wave functions must have purely for reasons of symmetry and irrespective of their explicit analytical form. However, this discussion of the symmetry aspects of ligand field theory would be artificial and unsatisfying without some... 

David Danovich was born in Russia in 1959. He graduated in physics from Irkutsk State University in Russia in 1982 and obtained his Ph.D. degree in Quantum Chemistry there in 1989 under the guidance of Dr. V. Zakrzevski and Prof. V. Voronov. He was a postdoctoral fellow with Professor Yitzhak Apeloig at Technion, Haifa, Israel, from 1990 to 1992. In 1992 he joined the group of Professor Sason Shaik at The Hebrew University, where he is now Computational Chemist at the Lise Meitner Minerva Center for Computational Quantum Chemistry. His major topics of interest are the design and implementation of codes for quantum mechanical computations and development of theoretical models for chemical systems. 

S.-F. Wu, R.D. Levine, Quantum mechanical computational studies of chemical reactions ... 

The electronic structures of the l,3,2(A2)-diazasiloles 83-85 and 89 have been probed by He(i) and He(n) photoelectron spectroscopy. The experimental ionization potentials are summarized in Table 3. The molecular orbitals were assigned with the help of quantum mechanical computations for model compounds and in each case the HOMO was found to be a Jt-type MO of b, symmetry. In the case of the benzol] and pyiido[/4 fused compounds 85 and 89 also the HOMO-1 is a Jt-type MO. The next band was assigned to a lone pair-type MO of aj symmetry, which is mostly located at the dicoordinated silicon atom. Therefore, the HOMOs of the cyclic silylenes 83 and 84 differ in nature from those of the homologues V-heterocyclic carbenes which consist of an essentially aj lone pair-type MO at... 

V. Thery, D. Rinaldi, J. L. Rivail, B. Maigret and G. G. Ferenczy, Quantum-mechanical computations on very large molecular-systems - the local self-consistent-field method, J. Comput. Chem., 15 (1994) 269-282. 

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