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An automated method for systematic hierarchical fragmentation of large molecules is presented utilizing isodesmic reactions. It is shown that by taking appropriate linear combination of fragment energies, it is possible to obtain highly accurate total energies when compared to total energy of full molecule.

An extensive study on the harmonic frequencies of a large set of small polyatomic closed-shell molecules computed at both single level ab initio and composite approximations is presented. Using various combinations of basis sets, composite methods are capable of predicting single level ab initio harmonic frequencies, which suggest a computationally affordable means of obtaining highly accurate vibrational frequencies.

The application of Collins' method of interpolating a potential energy surface to a bound state problem is described. It is used to obtain an accurate ground state zero-point energy, the vibrationally average rotational constants and the vibrationally averaged internal coordinates.

Research is presented concerning the detection of diketopiperazine's microwave spectrum in the frequency range of 48-72 GHz. The deviation of the microwave spectrum from that of a rigid rotor is discussed.

The fast-scan submillimeter spectroscopy technique (FASSST) is an instrument that provides great speed and improved analytical and detection capabilities. FASSST is based on the last part of the electromagnetic spectrum, the submillimeter, that is electronically tunable sources of high spectral brightness and spectral purity. Aside from its sensitivity, the FASSST instrument is also faster and less complex than other submillimeter systems.

First principles NMR calculations of the nuclear magnetic shielding tensor were performed. It is shown that by utilizing the fragmentation approach molecular property for a large class of molecules allows construction of a database of molecular fragments that could be used in NMR characterization of molecules.

The isomers of hydroxyacetaldehyde were studied by both two and eighteen dimensional quantum diffusion Monte Carlo (DMC) calculations. A total of four unique minima, and the transition states connecting them were observed and the results obtained indicate that not all of the isomers can be isolated in the gas phase.

The elemental frequencies of vibration of first-row closed-shell diatomics are estimated using both the levels of the ab initio theory. The results indicate that accurate fundamental frequencies can be computed economically and effectively using a precise composite method.

Examines the ab initio potential energy surface for the quartet electronic state of NH[sup +][sub 3] from the collision reaction of N([sup 4]S) and H[sup +][sub 3]. Detailed exploration of the ab initio potential energy surfaces; Analysis on the reaction paths and simulation of reactions; Investigation on the possibility of surface crossing.

Examines a modified form of Shepard interpolation of ab initio molecular potential energy surfaces. Improvements in accuracy over previous related schemes; Taylor expansion used in the interpolation formula; Parameters determining the confidence volume.

Examines the adiabatic potential energy surfaces (PES) for the ground and excited states of triplet methyl cation. Integration of molecular potential energy surfaces with multi-configurational self-consistent field methods; Investigation on the accuracy of PES; Properties of the reaction products.

Describes the development of a fast scan submillimeter spectroscopic technique which uses a voltage tunable backward wave oscillator (BWO) as a primary source of radiation. Basic spectroscopic system; Key system elements; System attributes; Tube characteristics; Frequency calibration scheme; Power supply scheme; BWO frequency-voltage characteristic.

Interpolated ab initio potential energy surfaces which describe abstraction and exchange reactions in collisions of hydrogen and water are reported. The electronic structure calculations are performed at the QCISD(T) level of theory, with an additivity approximation. A sufficiently large basis set is required to describe the Rydberg character of the electronic state for molecular configurations which are important for the exchange process. Classical and quantum dynamics calculations on the surfaces are presented. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The CH3 + +H2 →CH5 + + radiative associative reaction plays a major role in the synthesis of complex hydrocarbons in interstellar dust clouds and was studied using a part-classical, part-ab initio approach. A first-principles potential energy surface (PES) at CCSD/6-311++G(2df, 2pd) was constructed from a set of 828 data points in configurational space using modified Shepard interpolation, and was used in conjuction with long-range perturbation theory at large intermolecular distances. Data points are chosen so as to optimize PES construction according to the GROW software algorithm. Classical reaction dynamics was employed to infer the reaction rate coefficient. [Copyright &y& Elsevier]