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Wu Y.-N.,Quantum Theory Project | Kebaili N.,University Paris - Sud | Cheng H.-P.,Quantum Theory Project | Cahuzac P.,University Paris - Sud | And 2 more authors.
Journal of Chemical Physics | Year: 2012

To understand the role of chlorine in the stability and the observed fragmentation of Ag dendritic nanostructures, we have studied computationally two model systems using density functional theory. The first one relates to diffusion of Ag n and Ag n Cl m (n 1-4) clusters on an Ag (111) surface, and the second demonstrates interaction strength of (Ag 55)2 dimers with and without chloridation. Based on our calculated energy barriers, Ag n Cl m clusters are more mobile than Ag n clusters for n 1-4. The binding energy between two Ag 55 clusters is significantly reduced by surface chloridation. Bond weakening and enhanced mobility are two important mechanisms underlying corrosion and fragmentation processes. © 2012 American Institute of Physics.


Claudino D.,Quantum Theory Project | Gargano R.,University of Brasilia | Carvalho-Silva V.H.,State University of Goiás | E Silva G.M.,University of Brasilia | And 2 more authors.
Journal of Physical Chemistry A | Year: 2016

The present paper concludes our series of kinetics studies on the reactions involved in the complex mechanism of nitrogen trifluoride decomposition. Two other related reactions that, along with this mechanism, take part in an efficient boron nitride growth process are also investigated. We report results concerning two abstraction reactions, namely NF2 + N ⇄ 2NF and NF3 + NF ⇄ 2NF2, and two dissociations, N2F4 ⇄ 2NF2 and N2F3 ⇄ NF2 + NF. State-of-the-art electronic structure calculations at the CCSD(T)/cc-pVTZ level of theory were considered to determine geometries and frequencies of reactants, products, and transition states. Extrapolation of the energies to the complete basis set limit was used to obtain energies of all the species. We applied transition state theory to compute thermal rate constants including Wigner, Eckart, Bell, and deformed theory corrections in order to take tunneling effects into account. The obtained results are in good agreement with the experimental data available in the literature and are expected to provide a better phenomenological understanding of the NF3 decomposition role in the boron nitride growth for a wide range of temperature values. © 2016 American Chemical Society.


Drozd G.T.,Carnegie Mellon University | Melnichuk A.,Quantum Theory Project | Donahue N.M.,Carnegie Mellon University
Journal of Chemical Physics | Year: 2010

The absorption cross section of HOOH, a starting point for larger ROOH, was calculated using the "Wigner method." Calculations use the Wigner transform of ground state wave functions and classical approximations for excited state wave functions. Potential energy and transition dipole moment surfaces were calculated using the equation-of-motion coupled-cluster singles and doubles method over an extended Franck-Condon region. The first two O-O stretches and the first five HOOH torsional levels are included. This study also addresses two fundamental questions about ROOH photodissociation. The long wavelength à 1 A: B̃ 1 B excited state preference has been measured from dynamics experiments, but a Franck-Condon overlap explanation has not been directly verified. A moderate barrier to HOOH torsional motion and excited state dynamics affect the temperature dependence in the UV spectrum. Based on these initial findings for HOOH, photodissociation of large ROOH cannot be eliminated as an important factor for ozone and particulate matter production seen in both ambient and laboratory studies. © 2010 American Institute of Physics.


Molt Jr. R.W.,Quantum Theory Project | Watson Jr. T.,Quantum Theory Project | Lotrich V.F.,Quantum Theory Project | Bartlett R.J.,Quantum Theory Project
Journal of Physical Chemistry A | Year: 2011

The geometries, harmonic frequencies, electronic excitation levels, and energetic orderings of various conformers of RDX have been computed at the ab initio MP2 and CCSD(T) levels, providing more reliable results than have been previously obtained. We observe that the various local minimum-energy conformers are all competitive for being the absolute minimum and that, at reasonable temperatures, several conformers will appreciably contribute to the population of RDX. As a result, we have concluded that any mechanistic study to investigate thermal decomposition can reasonably begin from any one of the cyclohexane conformers of RDX. As such, it is necessary to consider the transition states for each RDX conformer to gauge what the activation energy is. Homolytic bond dissociation has long been speculated to be critical to detonation; we report here the most accurate estimates of homolytic BDEs yet calculated, likely to be accurate within 3 kcal mol-1. The differences in energy for homolytic BDEs among all the possible RDR conformers are again small, such that most all of the conformers can reasonably be speculated as the next step in the mechanism starting from the RDR radical. © 2011 American Chemical Society.


Cheng H.-P.,Quantum Theory Project
Molecular Physics | Year: 2010

In this paper, a few problems based on my work at QTP are selected and organized with a focus on physical systems and processes that involve interaction between extended states in solids and localized states in molecules. Such interactions are ubiquitous in interfacial processes that stir an intense interest in the science community. © 2010 Taylor & Francis.


Molt Jr. R.W.,Quantum Theory Project | Bazante A.,Quantum Theory Project | Watson Jr. T.,Quantum Theory Project | Bartlett R.J.,Quantum Theory Project
Journal of Molecular Modeling | Year: 2013

We have addressed the accuracy of calculating the enthalpy of formation of an arbitrary single reference molecule using practical ab initio methodologies. It is known that MP2 geometries with a triple zeta basis set are almost as reliable as CCSD(T) geometries. It is also known that CCSD(T) correlation energies, with basis extrapolation, feature chemical accuracy for single-reference molecules. We investigate what accuracy one might expect in enthalpies of formation from a MP2 geometry, MP2 harmonic vibrational frequencies, a CCSD(T) correlation energy using triple zeta basis sets. It is far from obvious, a priori, as to which error source contributes most significantly. We observe that the accuracy in calculating enthalpies of formation of single-reference molecules with this protocol is 4 kcal mol -1; our error analysis shows this comes almost exclusively from the correlation energy basis extrapolation, rather than errors intrinsic to MP2. © 2012 Springer-Verlag Berlin Heidelberg.


Molt Jr. R.W.,Quantum Theory Project | Watson T.,Quantum Theory Project | Bazante A.P.,Quantum Theory Project | Bartlett R.J.,Quantum Theory Project
Journal of Physical Chemistry A | Year: 2013

The octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocine (HMX) molecule is a very commonly studied system, in all 3 phases, because of its importance as an explosive; however, no one has ever attempted a systematic study of what all the major gas-phase conformers are. This is critical to a mechanistic study of the kinetics involved, as well as the viability of various crystalline polymorphs based on the gas-phase conformers. We have used existing knowledge of basic cyclooctane chemistry to survey all possible HMX conformers based on its fundamental ring structure. After studying what geometries are possible after second-order many-body perturbation theory (MBPT(2)) geometry optimization, we calculated the energetics using coupled cluster singles, doubles, and perturbative triples (CCSD(T))/cc-pVTZ. These highly accurate energies allow us to better calculate starting points for future mechanistic studies. Additionally, the plethora of structures are compared to existing experimental data of crystals. It is found that the crystal field effect is sometimes large and sometimes small for HMX. © 2013 American Chemical Society.


Gregusova A.,Quantum Theory Project | Perera S.A.,Quantum Theory Project | Bartlett R.J.,Quantum Theory Project
Journal of Chemical Theory and Computation | Year: 2010

Benchmark CCSD(T) 15N NMR calculations are performed for 35 experimentally known 15N shifts of 29 molecules. For the eight known gas phase experimental values of N2, HCN, CH3CN, NNO, NH3, NNO, (CH3)3N, and CH3NH 2, CCSD(T) with a basis set previously calibrated for 13C shifts is accurate to 0.2-3 ppm except for the NNO shift, which shows a deviation of 6 ppm. However, the differences between the computed and experimental values in solution due to solvent and finite temperature effects can be as large as ∼25 ppm and must be estimated to relate gas phase 0 K computed values to experiment. An empirical correction is obtained by studying the variations between the estimated solvent effects and the absolute shielding constant. It is shown that the average deviation of computed shifts falls to 3.6 ppm from 12.6 ppm when the correction is applied. © 2010 American Chemical Society.


PubMed | Quantum Theory Project
Type: Journal Article | Journal: The journal of physical chemistry. A | Year: 2011

The geometries, harmonic frequencies, elec-tronic excitation levels, and energetic orderings of various conformers of RDX have been computed at the ab initio MP2 and CCSD(T) levels, providing more reliable results than have been previously obtained. We observe that the various local minimum-energy conformers are all competitive for being the absolute minimum and that, at reasonable temperatures, several conformers will appreciably contribute to the population of RDX. As a result, we have concluded that any mechanistic study to investigate thermal decomposition can reasonably begin from any one of the cyclohexane conformers of RDX. As such, it is necessary to consider the transition states for each RDX conformer to gauge what the activation energy is. Homolytic bond dissociation has long been speculated to be critical to detonation; we report here the most accurate estimates of homolytic BDEs yet calculated, likely to be accurate within 3 kcal mol(-1). The differences in energy for homolytic BDEs among all the possible RDR conformers are again small, such that most all of the conformers can reasonably be speculated as the next step in the mechanism starting from the RDR radical.

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