Lawrence B. Harding

Bio: Lawrence B. Harding is an academic researcher from Argonne National Laboratory. The author has contributed to research in topics: Ab initio & Potential energy surface. The author has an hindex of 57, co-authored 162 publications receiving 9331 citations. Previous affiliations of Lawrence B. Harding include Princeton University & Environmental Molecular Sciences Laboratory.

The roaming atom: straying from the reaction path in formaldehyde decomposition

Abstract: We present a combined experimental and theoretical investigation of formaldehyde (H2CO) dissociation to H2 and CO at energies just above the threshold for competing H elimination. High-resolution state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H2. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H2. Quasi-classical trajectory calculations performed on a global potential energy surface for H2CO suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.

On the Enthalpy of Formation of Hydroxyl Radical and Gas-Phase Bond Dissociation Energies of Water and Hydroxyl

Abstract: In a recent letter (J. Phys. Chem. A, 2001, 105,1), we argued that, although all major thermochemical tables recommend a value of (OH) based on a spectroscopic approach, the correct value is 0.5 kcal/mol lower as determined from an ion cycle. In this paper, we expand upon and augment both the experimental and theoretical arguments presented in the letter. In particular, three separate experiments (mass-selected photoionization measurements, pulsed-field-ionization photoelectron spectroscopy measurements, and photoelectron-photoion coincidence measurements) utilizing the positive ion cycle to derive the O−H bond energy are shown to converge to a consensus value of the appearance energy AE0(OH+/H2O) = 146117 ± 24 cm-1 (18.1162 ± 0.0030 eV). With the most accurate currently available zero kinetic energy photoionization value for the ionization energy IE(OH) = 104989 ± 2 cm-1, corroborated by a number of photoelectron measurements, this leads to D0(H−OH) = 41128 ± 24 cm-1 = 117.59 ± 0.07 kcal/mol. This corres...

The Description of Chemical Bonding From AB Initio Calculations

Abstract: Concepts, such as hybridization and electronegativity, developed by Linus Pauling (1), Robert Mulliken (2), John Slater (3), and others in the 1930s have been powerful in rationalizing and predicting molecular structure, bond energies, and some aspects of reactivity. The power of these concepts is exemplified in the classic exposition, Nature of the Chemical Bond, by Linus Pauling (4). In recent years experimental and theoretical studies of numerous radicals have provided an assembly of quantitative information concerning bond energies, excitation energies, relative ordering of states, and shapes of potential curves, much of which is not explained by the older ideas. However, it has recently become possible to abstract from ab initio calculations qualitative concepts that rationalize many of the observed properties in such a way as to allow quantitative predictions for related systems. Currently the basis and application of this approach is distributed over a number of papers (5-9). Here we draw these ideas together with applications to number of related systems so as to indicate the utility and force of these methods. For simplicity of presentation we use Si and its hydrides (SiH, SiH_2, SiH_3, SiH_4) as prototypes for outlining the various concepts. These ideas are then extended to other molecules by replacing Si with Be through F and Mg through C1, and other related nontransition metal elements, and by replacing H with halogens such as F and C1.

Ab initio calculations of electronic and vibrational energies of HCO and HOC

Abstract: The ab initio calculation of electronic energies for numerous configurations of HCO and HOC, and a novel method for fitting the energies to a global surface are reported This surface is used to calculate all the bound vibrational states of nonrotating HCO and DCO using the Watson Hamiltonian Some quasibound vibrational states are also reported for nonrotating HOC for energies below the HOC saddle point energy Comparisons of the HCO and DCO vibrational energies are made with recent experimental results

Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: Characterization and correction of the time-dependent local density approximation ionization threshold

Abstract: This paper presents an evaluation of the performance of time-dependent density-functional response theory (TD-DFRT) for the calculation of high-lying bound electronic excitation energies of molecules. TD-DFRT excitation energies are reported for a large number of states for each of four molecules: N2, CO, CH2O, and C2H4. In contrast to the good results obtained for low-lying states within the time-dependent local density approximation (TDLDA), there is a marked deterioration of the results for high-lying bound states. This is manifested as a collapse of the states above the TDLDA ionization threshold, which is at ??HOMOLDA (the negative of the highest occupied molecular orbital energy in the LDA). The ??HOMOLDA is much lower than the true ionization potential because the LDA exchange-correlation potential has the wrong asymptotic behavior. For this reason, the excitation energies were also calculated using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the self-consistent field step. This was found to correct the collapse of the high-lying states that was observed with the LDA. Nevertheless, further improvement of the functional is desirable. For low-lying states the asymptotic behavior of the exchange-correlation potential is not critical and the LDA potential does remarkably well. We propose criteria delineating for which states the TDLDA can be expected to be used without serious impact from the incorrect asymptotic behavior of the LDA potential

Advances in methods and algorithms in a modern quantum chemistry program package

Abstract: Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

Bond Dissociation Energies of Organic Molecules

Abstract: In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

Bond dissociation energies of organic molecules

Abstract: In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

HITEMP, the high-temperature molecular spectroscopic database

Abstract: A new molecular spectroscopic database for high-temperature modeling of the spectra of molecules in the gas phase is described. This database, called HITEMP, is analogous to the HITRAN database but encompasses many more bands and transitions than HITRAN for the absorbers H2O, CO2, CO, NO, and OH. HITEMP provides users with a powerful tool for a great many applications: astrophysics, planetary and stellar atmospheres, industrial processes, surveillance, non-local thermodynamic equilibrium problems, and investigating molecular interactions, to name a few. The sources and implementation of the spectroscopic parameters incorporated into HITEMP are discussed.