Results 1 - 10 of 9535
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[en] A new point of view on the spectroscopic determination of potential energy surfaces (PES) is presented which is explicitly time dependent. A goal is to achieve an economic theoretical desciption which reduces to a bare minimum the information needed. The photoabsorption formula, photodissoci- ation, wavepacket calculations, overlaps, Raman scattering, overtone spectro- scopy, vibrational structure, mulecular spectra, periodic orbits are all included. The semiclassical approach used offers intuitive and computational advantages, and suggests new experiments designed to test PES features and dynamics
[en] An efficient procedure based on the reproducing kernel Hilbert space interpolation method is presented for constructing intermolecular potential energy surfaces (PES) using not only calculated ab initio data but also a priori information on long-range interactions. Explicitly, use of the reciprocal power reproducing kernel on the semiinfinite interval [0,∞) yields a set of exact linear relations between dispersion (multipolar) coefficients and PES data points at finite internuclear separations. Consequently, given a combined set of ab initio data and the values of dispersion (multipolar) coefficients, the potential interpolation problem subject to long-range interaction constraints can be solved to render globally smooth, asymptotically accurate ab initio potential energy surfaces. Very good results have been obtained for the one-dimensional He-He potential curve and the two-dimensional Ne-CO PES. The construction of the Ne-CO PES was facilitated by invoking a new reproducing kernel for the angular coordinate based on the optimally stable and shape-preserving Bernstein basis functions. (c) 2000 American Institute of Physics
[en] The rationale for the polynomial root interpolation method is presented. The formulation of interpolators and their properties are given. A summary of surfaces fit by these interpolators is included. As an example, the appli- cation of the fitting procedure to a potential energy surface for the ground electronic state of water is calculated by the CNDO/2 method. Interpolation by polynomial roots shows promise, but has not yet been demonstrated to be a viable procedure. The problems still to be addressed are listed
[en] Variational transition-state theory (VTST) is used to help identify the regions of potential energy surfaces (PES) that are most important in determining reliable rate constants. A wide variety of model PES are studied to predict general trends for various reaction types. The relationship between classical and quantum mechanical rate calculations is discussed, using the generalized transition-state theory (GTST). The classical and quantal-ground- state endoergicities and the vibrationally adiabatic ground-state (VAG) barrier heights are summarized. The application of VTST to three-dimensional reactions uses the canonical variational theory (CVT) and improved canonical variational theory (ICVT). The RMBEBO model was used to generate a wide variety of PES. Reactions were surveyed on RMBEBO and other surfaces. The rate constant calculated by applying the transition state assumption at the dynamical bottleneck of a reaction is often quite different from that calculated by conventional TST. In many cases, one can estimate what kind and size of effects will be important for a reaction with given saddle point properties and masses
[en] The potential energy surfaces for the formation of cytosine (Cyt) and a protonated cytosine (CytH+) from reactions of urea with cyanoacetylene (CA), cyanoacetaldehyde (CAA), or their protonated ions, CAAH+ and CAH+ with or without H2O, have been determined from quantum chemical calculation using the CBS-QB3 method. The overall activation energies of the formation of Cyt or CytH+ from urea + CA, urea + CAA and, urea + CAH+ are 127, 211, and 31 kJ mol−1, respectively, which are too high for the thermal reactions to occur in the interstellar medium (ISM). The barrierless reaction pathways have been proposed for the formation of CytH+ from urea + CAH++ H2O and urea + CAAH+. A kinetic analysis shows that the synthesis of Cyt through the formation of CytH+ from urea, CAH+, and H2O would be possible in the ISM.
[en] Here, the contemporaneous pointwise product of convective available potential energy (CAPE) and precipitation is shown to be a good proxy for lightning. In particular, the CAPE × P proxy for lightning faithfully replicates seasonal maps of lightning over the contiguous United States, as well as the shape, amplitude, and timing of the diurnal cycle in lightning. Globally, CAPE × P correctly predicts the distribution of flash rate densities over land, but it does not predict the pronounced land-ocean contrast in flash rate density; some factor other than CAPE or P is responsible for that land-ocean contrast.
[en] In this paper, a new spectroscopic and analytical potential energy function (APEF) investigation on the electronic state of the Na85Rb molecule is reported. This research is conducted by using the 4-terms variation algebraic energy consistent method (VAECM(4)) with a variable parameter λ. The calculated full vibrational energies of 78 levels are used in a global linear reduction to molecular vibrational constants. With these new and improved vibrational constants, the potential energy curve and the vibrational force constants have been calculated. Precise values for the long-range parameters and the dissociation energy have been derived. The results obtained in this study also provide valuable reference data for other areas of researches on Na85Rb. (paper)
[en] In this reply we respond to comments made by Repetto et al and by Butokov on our letter (Burko 2010 Eur. J. Phys. 31 L71-7), in which we discussed two different results for the elastic potential energy of a string element. One derived from the restoring force on a stretched string element and the other from the work done to bring the string to a certain distorted configuration. We argue that one cannot prefer from fundamental principles the former over the latter (or vice versa), and therefore one may apply either expression to situations in which their use contributes to insight. The two expressions are different by a boundary term which has a clear physical interpretation. For the case of standing waves, we argue that the latter approach has conceptual clarity that may contribute to physical understanding. (letters and comments)
[en] Ab initio molecular orbital (MO) methods are used to provide accurate descriptions of potential energy surfaces for chemical reactions. Thus, far, this has been limited with respect to systems having four or more atoms. However, computer calculation of the energy gradiant with respect to nuclear coordinates is now coming into use. In this review, the analytical calculation of the energy gradient is summarized. The methods of determining geometries, force constants, and normal vibrations for both equilibrium configurations and saddle points are discussed. The intrinsic reaction coordinate is also calculated. Examples of calculations are given