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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 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 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.

Abstract: In this work, we study the hydration free energies of butane, zwitterionic alanine, valine, serine, threonine, and asparagine, and two neuraminidase inhibitors by means of Monte Carlo (MC) simulation. The solute molecule, represented in the form of distributed multipoles and modified 6–12 potential, was varied from a non-interacting ‘ghost’ molecule to its full potential functions in TIP4P water. Intermediate systems with soft-core solute–solvent interaction potentials are simulated separately and then subjected to Bennett’s Acceptance ratio (BAR) for the free energy calculation. Hydration shells surrounding the solute particles were used to assess the quality of potential functions. [Copyright &y& Elsevier]

Abstract: In the energy-based fragmentation approach to computing the interaction energy between two molecules many interaction energies between small fragment molecules are required. Tens of thousands of these interactions are required if one of the molecules is as large as an enzyme. In this work we report the fragmentation of an entire enzyme and substrate and show that the majority of the small fragment molecule interactions can be accurately evaluated without the need to compute the ab initio interaction energy. This approach saves enormous amounts of cpu time and allows even more rapid evaluation of the total fragmentation interaction energy. [Copyright &y& Elsevier]

Abstract: The pure rotational spectra of the ground and five excited vibrational states of pyridine were measured, assigned and fit in the 75–110 and 260–370GHz frequency bands. An improved set of spectroscopic constants was obtained for the ground state, and all the rotational and quartic centrifugal distortion constants were obtained for the excited vibrational states. [Copyright &y& Elsevier]

Collins' method of interpolating a potential energy surface (PES) from quantum chemical calculations for reactive systems (Jordan, M. J. T.; Thompson, K. C.; Collins, M. A. J. Chem. Phys. 1995, 102, 5647. Thompson, K. C.; Jordan, M. J. T.; Collins, M. A. J. Chem. Phys. 1998, 108, 8302. Bettens, R. P. A.; Collins, M. A. J. Chem. Phys. 1999, 111, 816) has been applied to a bound state problem. The interpolation method has been combined for the first time with quantum diffusion Monte Carlo calculations to obtain an accurate ground state zero-point energy, the vibrationally average rotational constants, and the vibrationally averaged internal coordinates. In particular, the system studied was fluoromethane using a composite method approximating the QCISD(T)/6-311++G(2df,2p) level of theory. The approach adopted in this work (a) is fully automated, (b) is fully ab initio, (c) includes all nine nuclear degrees of freedom, (d) requires no assumption of the functional form of the PES, (e) possesses the full symmetry of the system, (f) does not involve fitting any parameters of any kind, and (g) is generally applicable to any system amenable to quantum chemical calculations and Collins' interpolation method. The calculated zero-point energy agrees to within 0.2% of its current best estimate. A[sub 0] and B[sub 0] are within 0.9 and 0.3%, respectively, of experiment. [ABSTRACT FROM AUTHOR]

Reports on the detection of the microwave spectrum of the smallest cyclic peptide called diketopiperazine. Failure to adopt the highly symmetric C2h planar ring structure in isolation; X-ray crystallography; Comparison of the derived rotational constants with those obtained from an ab initio molecular orbital calculation.

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]

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.

Abstract: Previous work involving the rotational spectrum of aniline was limited to the lower frequencies of 8–40GHz with very few lines being assigned. This work extends the earlier studies. Here we present a much more extensive measurement and assignment of the rotational spectrum of aniline in the frequency range of 75–110GHz. The observed frequencies have been assigned to the ground (0+), first (0−), and second excited (1+) states in the inversion vibration. With the newly assigned lines, significantly improved rotational constants and all five centrifugal distortion constants have been obtained. [Copyright &y& Elsevier]