Albert F. Wagner

Variational transition state theory and tunneling for a heavy–light–heavy reaction using an ab initio potential energy surface. 37Cl+H(D) 35Cl→H(D) 37Cl+35Cl

Ab initio POL–CI calculations, augmented by a dispersion term, are used to predict the potential energy surface for the reaction Cl+HCl. The saddle point is found to be nonlinear. The surface is represented by a rotated‐Morse‐oscillator spline fit for collinear geometries plus an analytic bend potential. Variational transition state theory calculations, based on a linear reference path, are carried out, and they yield much smaller rate constants than conventional transition state theory, confirming that earlier similar results for this heavy–light–heavy mass combination were consequences of the small skew angle and were not artifacts of the more approximate potential energy surfaces used in those studies. Transmission coefficients are calculated using approximations valid for large‐reaction‐path curvature and the potential along the reference path is scaled so that the calculated rate constant agrees with experiment. The resulting surface is used to compute the H/D kinetic isotope effect which is in quali...

Active Thermochemical Tables: thermochemistry for the 21st century

Active Thermochemical Tables (ATcT) are a good example of a significant breakthrough in chemical science that is directly enabled by the US DOE SciDAC initiative. ATcT is a new paradigm of how to obtain accurate, reliable, and internally consistent thermochemistry and overcome the limitations that are intrinsic to the traditional sequential approach to thermochemistry. The availability of high-quality consistent thermochemical values is critical in many areas of chemistry, including the development of realistic predictive models of complex chemical environments such as combustion or the atmosphere, or development and improvement of sophisticated high-fidelity electronic structure computational treatments. As opposed to the traditional sequential evolution of thermochemical values for the chemical species of interest, ATcT utilizes the Thermochemical Network (TN) approach. This approach explicitly exposes the maze of inherent interdependencies normally ignored by the conventional treatment, and allows, inter alia, a statistical analysis of the individual measurements that define the TN. The end result is the extraction of the best possible thermochemistry, based on optimal use of all the currently available knowledge, hence making conventional tabulations of thermochemical values obsolete. Moreover, ATcT offer a number of additional features that are neither present nor possible in the traditional approach. With ATcT, new knowledge can be painlessly propagated through all affected thermochemical values. ATcT also allows hypothesis testing and evaluation, as well as discovery of weak links in the TN. The latter provides pointers to new experimental or theoretical determinations that can most efficiently improve the underlying thermochemical body of knowledge.

A quasiclassical trajectory study of the OH+CO reaction

We present a quasiclassical trajectory study of the OH+CO reaction using a potential surface that has been derived from ab initio calculations. Among quantities that have been studied are cross sections for reaction and for HOCO complex formation, cross sections associated with reaction from excited vibrational and rotational states, product energy partitioning and CO2 vibrational‐state distributions, HOCO lifetime distributions, and thermal and state‐resolved rate constants. We also present the results of Rice–Ramsberger–Kassel–Marcus (RRKM) calculations, using the same potential‐energy surface, of HOCO lifetimes and of reactive and complex formation rate constants. The trajectory results indicate that the dominant mechanism for reaction involves complex formation at low energies. However, a direct reaction mechanism is responsible for half the reactive cross section at higher energies. This leads to a rate constant that is weakly temperature dependent at low temperatures, and becomes strongly temperatur...

Theoretical studies of fine-structure effects and long-range forces : potential-energy surfaces and reactivity of O(3P)+OH(2Π)

The role of fine structure in reactions without barriers in the potential‐energy surface is examined in general, and calculations are carried out for the specific case of O+OH→H+O2. The long‐range Hamiltonian, including electrostatic (dipole–quadrupole and quadrupole–quadrupole) and spin‐oribt interactions, is expressed in the asymptotic (separated species) basis for the 18 doubly degenerate states correlating to ground‐state reactants O(3P2,1,0)+OH(2Π3/2,1/2). Adiabatic potential‐energy surfaces are determined by diagonalization of the long‐range Hamiltonian. The adiabaticity of the reaction has been analyzed using general considerations about nonadiabatic processes and confirmed by direct integration of the coupled equations. The half collision through the coupling region is found to be predominantly adiabatic for the state correlating to reaction. Single‐surface reaction cross sections and rate constants have been obtained using the adiabatic capture, infinite‐order sudden approximation method. Our res...

Initiation in H2/O2: Rate constants for H2+O2→H+HO2 at high temperature

Abstract : The reaction between H2 and O2 has been studied in a reflected shock tube apparatus between temperatures of 1662 - 2097 K and pressures of 400 - 570 torr with Kr as the diluent gas. O atom atomic resonance absorption spectrometry (ARAS) was used to observe absolute [O]sub t under conditions of low [H2]sub 0 so that most secondary reactions were negligible. Hence, the observed [O]sub t was the direct result of the rate controlling reaction between H2 and O2. Three different reactions were considered, but experimental and ab initio theoretical results both indicated that the process, H2 + O2 -> H + HO2, is the most probable reaction. After rapid HO2 dissociation, O atoms are then instantaneously produced by H + O2 -> O + OH. Using the ab initio result, conventional transition state theoretical calculations (CTST) with tunneling corrections give the expression k(th/1) = 1.288 X 10(-18) T(2.4328) exp(-26,926 K/T) cu cm molecule(-1) s(-1), applicable between 400 and 2300 K. This theoretical result agrees with the present experimental determinations and those at lower temperature, derived from earlier work on the reverse reaction.

Exploring the OH+CO reaction coordinate via infrared spectroscopy of the OH–CO reactant complex

A hydrogen-bonded complex of OH with CO is identified along the reaction coordinate for the OH+CO↔HOCO→H+CO2 reaction. The existence of this linear OH–CO complex is established by infrared action spectroscopy, which accesses vibrational stretching and bending modes of the complex. Complementary electronic structure calculations characterize the OH–CO and OH–OC complexes, the transition state for HOCO formation, and the reaction pathways that connect these complexes directly to the HOCO intermediate.

Interpolating Moving Least-squares Methods for Fitting Potential Energy Surfaces: A Strategy for Efficient Automatic Data Point Placement in High Dimensions

An accurate and efficient method for automated molecular global potential energy surface (PES) construction and fitting is demonstrated. An interpolating moving least-squares (IMLS) method is developed with the flexibility to fit various ab initio data: (1) energies, (2) energies and gradients, or (3) energies, gradients, and Hessian data. The method is automated and flexible so that a PES can be optimally generated for trajectories, spectroscopy, or other applications. High efficiency is achieved by employing local IMLS in which fitting coefficients are stored at a limited number of expansion points, thus eliminating the need to perform weighted least-squares fits each time the potential is evaluated. An automatic point selection scheme based on the difference in two successive orders of IMLS fits is used to determine where new ab initio data need to be calculated for the most efficient fitting of the PES. A simple scan of the coordinate is shown to work well to identify these maxima in one dimension, but this search strategy scales poorly with dimension. We demonstrate the efficacy of using conjugate gradient minimizations on the difference surface to locate optimal data point placement in high dimensions. Results that are indicative of the accuracy, efficiency, and scalability are presented for a one-dimensional model potential (Morse) as well as for three-dimensional (HCN), six-dimensional (HOOH), and nine-dimensional (CH4) molecular PESs.

Interpolating moving least-squares methods for fitting potential energy surfaces: Detailed analysis of one-dimensional applications

We present the basic formal and numerical aspects of higher degree interpolated moving least-squares (IMLS) methods. For simplicity, applications of these methods are restricted to two one-dimensional (1D) test cases: a Morse oscillator and a 1D slice of the HN2→H+N2 potential energy surface. For these two test cases, we systematically examine the effect of parameters in the weight function (intrinsic to IMLS methods), the degree of the IMLS fit, and the number and placement of potential energy points. From this systematic study, we discover compact and accurate representations of potentials and their derivatives for first-degree and higher-degree (up to nine degree) IMLS fits. We show how the number of ab initio points needed to achieve a given accuracy declines with the degree of the IMLS. We outline automatic procedures for ab initio point selection that can optimize this decline.

Interpolating moving least-squares methods for fitting potential energy surfaces: computing high-density potential energy surface data from low-density ab initio data points.

A highly accurate and efficient method for molecular global potential energy surface (PES) construction and fitting is demonstrated. An interpolating-moving-least-squares (IMLS)-based method is developed using low-density ab initio Hessian values to compute high-density PES parameters suitable for accurate and efficient PES representation. The method is automated and flexible so that a PES can be optimally generated for classical trajectories, spectroscopy, or other applications. Two important bottlenecks for fitting PESs are addressed. First, high accuracy is obtained using a minimal density of ab initio points, thus overcoming the bottleneck of ab initio point generation faced in applications of modified-Shepard-based methods. Second, high efficiency is also possible (suitable when a huge number of potential energy and gradient evaluations are required during a trajectory calculation). This overcomes the bottleneck in high-order IMLS-based methods, i.e., the high cost/accuracy ratio for potential energy evaluations. The result is a set of hybrid IMLS methods in which high-order IMLS is used with low-density ab initio Hessian data to compute a dense grid of points at which the energy, Hessian, or even high-order IMLS fitting parameters are stored. A series of hybrid methods is then possible as these data can be used for neural network fitting, modified-Shepard interpolation, or approximate IMLS. Results that are indicative of the accuracy, efficiency, and scalability are presented for one-dimensional model potentials as well as for three-dimensional (HCN) and six-dimensional (HOOH) molecular PESs.

A comparative study of the reaction dynamics of several potential energy surfaces of O(3P)+H2→OH+H. I

Two new potential energy surfaces for th O+H2→OH+H reaction are presented, and a detailed comparision of the saddle point properties and thermal rate constants of these and of six other O+H2 surfaces is made. The two new surfaces are (1) an extended BEBO surface and (2) a rotated‐Morse‐oscillator‐spine (RMOS) fit to the accurate ab initio POLCI surface of Walch and Dunning. In the BEBO surface, an improved end atom repulsive potential is used which leads to a much more accurate barrier estimate (11.52 kcal/mol) than with the usual anti‐Morse expression. The POLCI–RMOS surface is an essentially quantitative fit to the ab initio points, and has a barrier of 12.58 kcal/mole. The other O+H2 surfaces examined include the LEPS surface of Westenberg and de Haas (LEPS‐WDH) and of Johnson and Winter (LEPS‐JW), the diatomics‐in‐molecules (DIM) surface of Whitlock, Muckerman, and Fisher, the ab initio surface of Howard, McLean, and Lester (HML), and a fit to HML’s surface by Schinke and Lester (SL). For the LEPS‐JW ...