Accurate ab initio potentials at low cost via correlation scaling and extrapolation: Application to CO(AΠ1)

Oxepin Derivatives Formation from Gas-Phase Catechol Ozonolysis

Quantum chemical calculations are performed to explore all of the possible pathways for primary ozonide (POZ) formation from gas-phase ozonolysis of catechol. Canonical transition state theory has been used to calculate the rate coefficients of individual steps for the formation of POZ. The calculated rate coefficients for 1,3-cycloaddition of ozone at the (i) unsaturated C(OH)═C(OH) bond and (ii) CH═C(OH) of catechol, respectively, are in good agreement with the experimental rate constant. In general, subsequent decomposition of POZ leads to well-known Criegee Intermediates. This work reveals a parallel pathway by which the endo-addition of ozone at CH═C(OH) of catechol proceeds through oxepin derivatives along with the paths leading to Criegee Intermediates and peroxy acids. The 7-membered heterocyclic oxepin derivatives have lower energies than Criegee Intermediates but similar relative energies with peroxy acids.

Globally Accurate Potential Energy Surface for BH2+2(13A') Using the Switching Function Formalism

A great number of ab initio energy points are calculated using the aug-cc-pV(Q,5)Z basis sets at the multireference configuration interaction level and extrapolated to the complete basis set limit. An exact three-dimensional potential energy surface of the ground-state BH₂⁺ is obtained. A switching function is developed to model the transition of B⁺(³P) to B⁺(¹S) to guarantee the reliable behavior at B⁺(³P) + H₂(X¹∑_g⁺) and BH⁺(X²∑⁺) + H(²S) dissociation limits. The various topographic features of the new global potential energy surface are discussed in detail, showing a good agreement with the previous results from the theory. The quasi-classical trajectory method is utilized to calculate the integral cross sections of the B⁺(³P) + H₂(X¹∑_g⁺) (v = 0, j = 0) → BH⁺(X²∑⁺) + H(²S) reaction, which can provide another support for reliability of the title potential energy surface.

Accurate Potential Energy Surface for Quartet State HN2 and Interplay of N(4S) + NH(X̃3Σ-) versus H + N2(A3Σu+) Reactions

A global potential energy surface for the lowest quartet state of HN₂ is reported for the first time from accurate multireference ab initio calculations extrapolated to the complete basis set limit using the double many-body expansion method. All its stationary points are characterized, with the lowest quartet of HN₂ predicted to have a bent global minimum 36 kcal mol^-1 below the N(⁴S) + NH(X̃³Σ^-) asymptote, from which it is barrierlessly achievable. The entire set of calculated ab initio points has been fitted for energies up to 1000 kcal mol^-1 above the global minimum with an RMSD of 0.89 kcal mol^-1, a gap comprising all identified stationary points. Special care is taken in modeling the involved long-range forces and cusps caused by crossing seams. The novel PES prompts for the calculation of rate constants for several unexplored reactions that are relevant for combustion, plasma, and atmospheric chemistry.

Accurate global potential energy surface by extrapolation to the complete basis set limit and dynamics studies for ground state of H2S. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;219:267-273. [PMID: 31048256 DOI: 10.1016/j.saa.2019.04.051]

The atomic ions with hydrogen reactions play important role in biological chemistry, atmospheric and combustion chemistry. In this work, a full three-dimensional global many-body expansion potential energy surface (PES) for the ground state of H₂S^-(²A') is reported for the first time, since there is almost no investigations can accurately and completely describe the PES. It is obtained using many-body expansion theory and an extensive set of accurate ab initio energies extrapolated to the complete basis set limit, for improving the accuracy of the potential energy surface. The topographical features of the new three-dimensional global potential energy surface are examined in detail, and found to be in good agreement with theoretical and experimental results. To better test the reaction, such a surface was applied to quasi classical trajectory calculations for S^-(²P)+H₂(¹Σ_g⁺)→SH^-(¹Σ)+H(²S) reaction. The integral cross sections, differential cross sections and the rate coefficients have been computed. Results indicate that such a work can be recommended for corresponding atmospheric dynamics studies and as building blocks for constructing the many body expansion potential energy surface of larger S^-/H containing systems.

Formation of Criegee intermediates and peroxy acids: a computational study of gas-phase 1,3-cycloaddition of ozone with catechol

A detailed theoretical investigation of gas-phase 1,3-cycloaddition of ozone with catechol is presented to explore the discrepancies in previous theoretical and experimental rate constants. DFT based PBE, TPSS, B3LYP, B3PW91, M06-2X, wB97XD, MN15 and high-level CCSD(T) methods are used for the calculation. Canonical transition state theory has been used to calculate the rate coefficients of individual steps. The calculated rate coefficients are compared with the experimental and previously calculated rate constant. The possible pathways for primary ozonide (POZ) formation and subsequent reactions to yield the Criegee Intermediates (CI) and peroxy acids (POA) are investigated. The endo-POZ may undergo conversion to exo-POZ or form the Creigee Intermediates. This work shows a novel pathway by which the exo-POZ can form more stable and chemically different species, peroxy acids, by abstracting an H atom from the OH group.

Accurate global potential energy surface for the ground state of CH2 + by extrapolation to the complete basis set limit

A full three-dimensional global potential energy surface is reported for the ground state of CH2 + by fitting accurate multireference configuration interaction energies calculated using aug-cc-pVQZ and aug-cc-pV5Z basis sets with extrapolation of the electron correlation energy to the complete basis set limit. The topographical characteristics have been compared in detail with a potential energy surface of the same type recently reported [J. Chem. Phys., 2015, 142, 124302] based on a least-squares fit to accurate high level ab initio MRCI(Q) energies, calculated using AV6Z basis set. The new three-dimensional global potential energy surface is then used in quasiclassical trajectory calculations for H(2S) + CH+(X 1Σ+) → C+(2P) + H2(X 1Σg +) reaction. The integral cross sections, differential cross sections and the rate coefficients have been computed. A comparison shows that our potential energy surface can be applied to any type of dynamic study.

Straightening the Hierarchical Staircase for Basis Set Extrapolations: A Low-Cost Approach to High-Accuracy Computational Chemistry

Because the one-electron basis set limit is difficult to reach in correlated post-Hartree-Fock ab initio calculations, the low-cost route of using methods that extrapolate to the estimated basis set limit attracts immediate interest. The situation is somewhat more satisfactory at the Hartree-Fock level because numerical calculation of the energy is often affordable at nearly converged basis set levels. Still, extrapolation schemes for the Hartree-Fock energy are addressed here, although the focus is on the more slowly convergent and computationally demanding correlation energy. Because they are frequently based on the gold-standard coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)], correlated calculations are often affordable only with the smallest basis sets, and hence single-level extrapolations from one raw energy could attain maximum usefulness. This possibility is examined. Whenever possible, this review uses raw data from second-order Møller-Plesset perturbation theory, as well as CCSD, CCSD(T), and multireference configuration interaction methods. Inescapably, the emphasis is on work done by the author's research group. Certain issues in need of further research or review are pinpointed.

CBS extrapolation in electronic structure pushed to the end: a revival of minimal and sub-minimal basis sets

The complete basis set (CBS) limit is secluded in calculations of electronic structure, and hence CBS extrapolation draws immediate attention.

Assessing How Correlated Molecular Orbital Calculations Can Perform versus Kohn-Sham DFT: Barrier Heights/Isomerizations

To assess the title issue, 38 hydrogen transfer barrier heights and 38 non-hydrogen transfer barrier heights/isomerizations extracted from extensive databases have been considered, in addition to 4 2 p-isomerization reactions and 6 others for large organic molecules. All Kohn-Sham DFT calculations have employed the popular M06-2X functional, whereas the correlated molecular orbital (MO)-based ones are from single-reference MP2 and CCSD(T) methods. They have all utilized the same basis sets, with raw MO energies subsequently extrapolated to the complete basis set limit without additional cost. MP2 calculations are found to be as cost-effective as DFT ones and often slightly more, while showing a satisfactory accuracy when compared with the reference data. Although the focus is on barrier heights, the results may bear broader implications, in that one may see successes and difficulties of DFT when compared with traditional MO theories for the same data.

Theoretical investigation of gas-phase molecular complex formation between 2-hydroxy thiophenol and a water molecule

A systematic study of the interaction energies and hydrogen bonding interaction of a gas-phase molecular complex between 2-hydroxy thiophenol and a water molecule.

Is HO3− multiple-minimum and floppy? Covalent to van der Waals isomerization and bond rupture of a peculiar anion

The HO₃⁻ anion is multiple-minimum and floppy: the two main isomers and isomerization barrier all lie quite below dissociation.

Accurate double many-body expansion potential energy surface by extrapolation to the complete basis set limit and dynamics calculations for ground state of NH2

An accurate single-sheeted double many-body expansion potential energy surface is reported for the title system. A switching function formalism has been used to warrant the correct behavior at the H2(X1Σg+)+N(2D) and NH (X3Σ-)+H(2S) dissociation channels involving nitrogen in the ground N(4S) and first excited N(2D) states. The topographical features of the novel global potential energy surface are examined in detail, and found to be in good agreement with those calculated directly from the raw ab initio energies, as well as previous calculations available in the literature. The novel surface can be using to treat well the Renner-Teller degeneracy of the 12A″ and 12A' states of NH 2. Such a work can both be recommended for dynamics studies of the N(2D)+H2 reaction and as building blocks for constructing the double many-body expansion potential energy surface of larger nitrogen/hydrogen-containing systems. In turn, a test theoretical study of the reaction N(2D)+H2(X1Σg+)(ν=0,j=0)→NH (X3Σ-)+H(2S) has been carried out with the method of quantum wave packet on the new potential energy surface. Reaction probabilities, integral cross sections, and differential cross sections have been calculated. Threshold exists because of the energy barrier (68.5 meV) along the minimum energy path. On the curve of reaction probability for total angular momentum J = 0, there are two sharp peaks just above threshold. The value of integral cross section increases quickly from zero to maximum with the increase of collision energy, and then stays stable with small oscillations. The differential cross section result shows that the reaction is a typical forward and backward scatter in agreement with experimental measurement result.

Potential curves for the dissociative recombination of CO+

Large scale ab initio calculations are reported for the diabatic (3)Π, (1)Π, (1)Σ(+), (1)Δ, (3)Σ(+), and (3)Δ valence states of CO that provide routes for the dissociative recombination of the ground electronic and vibrational state of CO(+). The most important routes are 2(3)Π, 3(3)Π, 2(1)Π, and D'(1)Σ(+). For electron energies below 0.2961 eV, from the v = 0 ion level, the first two states can generate excited atoms, O((1)D) and C((1)D), but the last two states yield only ground state atoms. From v = 0, hot ground state atoms are generated at 0 eV from each of the four states with C and O having 1.67 and 1.25 eV of kinetic energy, respectively. The potential curves are compared to prior calculations and experiments.

Guided-ion-beam and ab initio study of the Li+, K+, and Rb+ association reactions with gas-phase butanone and cyclohexanone in their ground electronic states

The association reactions between Li(+), K(+), and Rb(+) (M) and butanone and cyclohexanone molecules under single collision conditions have been studied using a radiofrequency-guided ion-beam apparatus, characterizing the adducts by mass spectrometry. The excitation function for the [M-(molecule)](+) adducts (in arbitrary units) has been obtained at low collision energies in the 0.10 eV up to a few eV range in the center of mass frame. The measured relative cross sections decrease when collision energy increases, showing the expected energy dependence for adduct formation. The energetics and structure of the different adducts have been calculated ab initio at the MP2(full) level, showing that the M(+)-molecule interaction takes place through the carbonyl oxygen atom, as an example of a nontypical covalent chemical bond. The cross-section energy dependence and the role of radiative cooling rates allowing the stabilization of the collision complexes are also discussed.