Quasi-classical Trajectory Method Research Articles

Mode-specific dynamics in multichannel reaction NH+ + H2.

The vibrational- and rotational-mode specificity of the multichannel NH+ + H2 reaction is studied on a recently constructed ab initio-based global potential energy surface using an initial state selected quasi-classical trajectory method, and the trajectories are analyzed using an isometric feature mapping and k-means approach. All excitation modes promote two reactions (R1: NH'+ + H2 → NH+ + HH' and R4: NH'+ + H2 → NH2+ + H') where both NH and HH bonds are broken, but reduce the reactivity of the proton-transfer reaction R2 (NH'+ + H2 → N + H'H2+) at low collision energies. For the hydrogen-transfer reaction R3 (NH'+ + H2 → HNH'+ + H), the rotational excitation of NH+ enhances the reactivity remarkably, while its vibrational excitation has an inhibiting effect on the reaction. The trajectory analyses show that the vibrational and rotational excitations of NH+ make R3 tend to go over a submerged saddle point instead of extracting hydrogen atoms directly. On the other hand, the motions of the H2 reactant facilitate the enhancement of the reactivity but they do not affect the mechanism of R3. In addition, the results suggest that the coupling of the isometric feature mapping and the k-means approach in the trajectory analysis is an appropriate tool for reaction-dynamics studies.

Quantum interference in the mechanism of H + LiH+ → H2 + Li+ reaction dynamics.

In this work, the detailed reaction mechanism of the astrochemically relevant exoergic and barrierless H + LiH+ → H2 + Li+ reaction is investigated by both time-dependent wave packet and quasi-classical trajectory (QCT) methods on the ab initio electronic ground state potential energy surface reported by Martinazzo et al. [Martinazzo et al., J. Chem. Phys., 2003, 119, 11241]. The interference terms due to the coherence between the partial waves are quantified. When plotted along the scattering angle they reveal interference of constructive or destructive nature. Significant interference was found in the differential cross-section (DCS) which is a symbolic of the non-statistical nature of the reaction. This is further complemented by calculating the lifetime of the collision complex by the QCT method. It is found that the reaction follows a direct stripping mechanism at higher collision energies and yields forward scattered products from collisions involving high total angular momentum. At low collision energies, the reaction follows a mixed direct/indirect mechanism but with a dominant indirect contribution. The product state-resolved DCSs reveal that two opposite mechanisms co-exist, both at low and high collision energies. The microscopic scattering mechanism of the reaction is found to be unaffected by the ro-vibrational excitation of the reagent diatom.

State-to-state dynamics of S+(2D) + H2(X1Σg+)(v, j) collision reaction based on the H2S+ (X2A′′)potential energy surface

The state-to-state dynamics of the S+(2D) + H2(X1Σg+)(v, j) collision reaction is investigated on the H2S+(X2A″) potential energy surface by using the quasi-classical trajectory (QCT) method. The obtained total integral cross section (ICS) and rate constant of the reaction display obvious vibrational excitation effect. The differential cross sections show the effect of vibrational excitation, collision energy and deep well of potential energy surface. A comparison of the state-to-state ICSs from QCT method with the ones from quantum mechanical method proves the validity of QCT calculations. The further exploration of state-resolved ICSs and rate constants for different vibrational quantum numbers of reactant indicates that (i) the higher rotational state of product is much easier to form; (ii) the width of product molecule excited state distribution is obviously enlarged by the vibrational excitation of reactant.

Final-State-Resolved Dynamics of the H3+ + CO → H2 +HCO+/HOC+ Reaction: A Quasi-Classical Trajectory Study.

The ion-molecule reaction H3+ + CO → H2 + HCO+/HOC+, which initiates the formation of crucial organic molecules, plays a key role in interstellar and circumstellar environments. In this work, the quasi-classical trajectory method is employed to study the reaction dynamics on a recently developed full-dimensional global potential energy surface (PES). The calculated product internal energy distributions and relative internal excited fractions agree reasonably well with the experimental measurements. For the two reaction channels, most of the available energy flows into the vibrational modes of HCO+ or HOC+ at low collision energies, followed by the translational mode and the rotational modes of HCO+ or HOC+. As the collision energy increases, the proportion of the product translational energy increases while the proportion of the product vibrational energy decreases. Furthermore, the CH and CO stretching modes and their combination bands are effectively excited for the product HCO+ while the bending mode is remarkably excited for the product HOC+.

Theoretical Investigations of Rate Coefficients for H + O3 and HO2 + O Reactions on a Full-Dimensional Potential Energy Surface.

In this work, a machine learning method is used to construct a high-fidelity multichannel global reactive potential energy surface (PES) for the HO3 system from 21452 high-level ab initio calculations at the explicitly correlated multireference configuration interaction (MRCI-F12) level of theory. The permutation invariance of the PES with respect to the three identical oxygen atoms is enforced using permutation invariant polynomials (PIPs) in the input layer of a neural network (NN). This PIP-NN representation is highly faithful to the ab initio points, with a root-mean-square error of 0.20 kcal/mol. Using this PES, the kinetics of H + O3 → OH + O2 (R1) and HO2 + O → OH + O2 (R2) reactions were investigated using a quasi-classical trajectory method over a wide temperature range (200-2000 K). It was found that the calculated thermal rate coefficients of R1 and R2, exhibiting positive and negative temperature dependences, respectively, are in reasonably good agreement with most experimental measured values. These temperature dependences can be attributed to the presence and absence of an entrance channel potential barrier.

Surface for methane combustion: O(3P) +CH4 → OH+CH3**Project supported by the National Natural Science Foundation of China (Grant No. 51574016) and completed while the author was in residence at UNSW, Australia supported by the International Cooperation Training Program for Innovative Talents of USTB.

Kinetic investigations including quasi-classical trajectory and canonical unified statistical theory method calculations are carried out on a potential energy surface for the hydrogen-abstraction reaction from methane by atom O(3P). The surface is constructed using a modified Shepard interpolation method. The ab initio calculations are performed at the CCSD(T) level. Taking account of the contribution of inner core electrons to electronic correlation interaction in ab initio electronic structure calculations, modified optimized aug-cc-pCVQZ basis sets are applied to the all-electrons calculations. On this potential energy surface, the triplet oxygen atom attacks methane in a near-collinear H–CH3 direction to form a saddle point with barrier height of 13.55 kcal/mol, which plays a key role in the kinetics of the title reaction. For the temperature range of 298–2500 K, our calculated thermal rate constants for the O(3P) + CH4 → OH + CH3 reaction show good agreement with relevant experimental data. This work provides detailed mechanism of this gas-phase reaction and a theoretical guidance for methane combustion.

Dynamics of H(2S) + CH(X2Π) reactions based on a new CH2() surface via extrapolation to the complete basis set limit

A grid of 5285 ab initio points is utilized to construct a 3D potential energy surface (PES) of the system. In the calculation procedure, the aug-cc-pVXZ (X = Q and 5) basis sets with Davidson correction are employed. The reference wave function for the multi-reference configuration interaction calculations is composed of a full valence complete-active-space self-consistent field wave function. In order to get a more accurate PES, the complete basis set (CBS) limit proposal and the many-body expansion form are used, with the total root mean square deviation of the final CBS-PES being 0.0349 eV. Based on the accurate CBS-PES, the stationary points and vibrational energy levels are obtained and examined in detail, which agree well with other theoretical results. Then, utilizing the CBS-PES and quasi-classical trajectory method, the integral cross-sections (ICSs) and rate constants of the → / reactions are calculated. It is found that the present ICSs are in good agreement with other theoretical results, and is the major product channel. For this channel, the rate constants calculated in this work agree well with experimental and other theoretical results in the high-temperature range. It is worth noting that at room temperature, the theoretical results, including the present work, are consistent with each other, but they are all higher (about 7–10 times) than the experimental result, which implies that a new measurement for the rate constant at room temperature is necessary.

Exploring reaction mechanism and vibrational excitation effect in H + CH([formula omitted] = 0) reaction

The reaction mechanism and vibrational excitation effect in H + CH → C + H2 reaction is analyzed in detail by utilizing quasi-classical trajectory method. The capture time, vibrational excitation effect for the total integral cross sections (ICSs), rate constants, and rovibrational state resolved ICSs are studied. Results show that with the collision energy increasing, the direct mechanism is becoming increasingly important, but, the indirect mechanism is still important and even dominant. The analysis of the ICSs according to the maximum impact parameter and probability lets us to get a deep insight into the effect of vibration excitation on reaction.

Dynamics studies of the Li(2S) + H2(X1Σg+) → LiH (X1Σ+) + H(2S) reaction by time-dependent wave packet and quasi-classical trajectory methods

Based on the potential energy surface reported by Yuan and co-workers (Phys. Chem. Chem. Phys. 17 (2015) 11732–11739), the specific state (v0 = 0, j0 = 0) dynamics calculations of the Li(2S) + H2(X1Σg+) → LiH(X1Σ+) + H(2S) reaction were performed using the time-dependent wave packet and quasi-classical trajectory methods in the collision energy range from 2.0 to 3.0 eV. The reaction probability, integral cross section, differential cross section, the distribution of LiH product and so on were reported at state-to-state level of theory. The results obtained by quasi-classical trajectory method are in good agreement with quantum values, which indicates that the quantum effect is not very apparent. The differential cross sections results indicate that the abstract reaction mechanism is dominant in the reaction.

Accurate global adiabatic potential energy surfaces for three low-lying electronic states of AlH2 free radicals.

In order to obtain the all-round molecular properties of the AlH2 system and the corresponding dynamical characteristics of the Al + H2 (v = 0, j = 0) → H + AlH reaction, three significant global adiabatic potential energy surfaces of AlH2 (X2A1, 2B1, and 2B2) free radicals were constructed for the first time. Ab initio energies were calculated under the multi-reference configuration interaction method and the aug-cc-pV(T,Q)Z basis sets; then the ab initio energies were extrapolated to the complete basis sets limit. The three adiabatic potential energy surfaces were constructed by the many-body expansion theory. The maximum root-mean square error was just 50 cm-1, which was small enough to ensure that the potential energy surfaces were accurate. The concerned T-type insertion topographical features, dissociation schemes, C2v geometry reaction mechanisms, and minimum energy curve paths were investigated and are discussed in detail. Several differences from previous studies are also pointed out. Eventually, the integral cross-sections of Al + H2 (v = 0, j = 0) → H + AlH reaction as calculated by quasi-classical trajectory method were employed to predict the dynamical properties of AlH2, providing the most reliable theoretical reference of the dynamical characteristics known thus far for such a reaction. These new potential energy surfaces can be treated as a reliable basis for investigation of the dynamics and as a component for constructing larger aluminum-/hydrogen-containing systems.

Effects of rovibrational excitation of LiH on the LiH depletion and H exchange channels for the reaction H ([formula omitted]S) + LiH (X[formula omitted]) on a new potential energy surface

The quasiclassical trajectory method is used to calculate the dynamics of the reaction H + LiH for both reaction channels. The initial state-selected and energy-resolved integral cross-sections, differential cross-sections, and state distributions of the products are investigated in the collision energy range 0.02–0.56 eV. Results show that both vibrational and rotational excitations reduce the reactivity of the LiH depletion (R1) channel but promote that of the H exchange (R2) channel. Forward scattering is prominent in the R1 channel, but backscattering is slightly favored in the R2 channel. The influence of vibrational excitation is reduced at higher energies.

The influence of the isotope substitution on the O + LiH+/LiD+ reactions

The influence of the isotope substitution on the O + LiH+ → Li+ + OH reaction is studied by employing the quasiclassical trajectory method. The integral cross sections, the differential cross sections and the vibrational state distributions of the molecule products are calculated to analyse the isotope effects. Moreover, two kinds of typical reactive trajectories of the title reactions are discussed in detail. According to the calculation results, we find that the influence of the isotope substitution on the O + LiH+ reaction is very sensitive to the collision energy and the mass combination of the reaction.

Effect of isotope on state-to-state dynamics for reactive collision reactions and in ground state 12A″ and first excited 12A′ potential energy surfaces**Project supported by the National Natural Science Foundation of China (Grant No. 11504206) and the Shandong Jiaotong University PhD Research Start-up Fund, China.

We carry out quantum scattering dynamics and quasi-classical trajectory (QCT) calculations for the reactive collision in the ground (12A″) and first excited (12A′) potential energy surface. We calculate the reaction probabilities of and reaction for total angular momentum J = 0. The results calculated by QCT are consistent with those from quantum mechanical wave packet. Using the QCT method, we generate in the center-of-mass frame the product state-resolved integral cross-sections (ICSs); two commonly used generalized polarization-dependent differential cross-sections (PDDCSs), (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt); and three angular distributions of the product rotational vectors, P(θr), P(ϕr), and P(θr, ϕr). We discuss the influence on the scalar and vector properties of the potential energy surface, the collision energy, and the isotope mass. Since there are deep potential wells in these two potential energy surfaces, their kinetic characteristics are similar to each other and the isotopic effect is not obvious. However, the well depths and configurations of the two potential energy surfaces are different, so the effects of isotopic substitution on the integral cross-section and the rotational polarization of product are different.

Dynamics of the Au + H2 reaction by time-dependent wave packet and quasi-classical trajectory methods

Dynamics of the Au + H2 reaction are studied using time-dependent wave packet (TDWP) and quasi-classical trajectory (QCT) methods based on a new potential energy surface [Int. J. Quantum Chem. 118 e25493 (2018)]. The dynamic properties such as reaction probability, integral cross section, differential cross section and the distribution of product are studied at state-to-state level of theory. Furthermore, the present results are compared with the theoretical studies available. The results indicate that the complex-forming reaction mechanism is dominated in the reaction in the low collision energy region and the abstract reaction mechanism plays a dominant role at high collision energies. Different from previous theoretical calculations, the side-ways scattering signals are found in the present work and become more and more apparent with increasing collision energy.

Ring Polymer Molecular Dynamics in Gas-Surface Reactions: Inclusion of Quantum Effects Made Simple.

Accurately modeling gas-surface collision dynamics presents a great challenge for theory, especially in the low-energy (or temperature) regime where quantum effects are important. Here, a path integral-based nonequilibrium ring polymer molecular dynamics (NE-RPMD) approach is adapted to calculate dissociative initial sticking probabilities (S0) of H2 on Cu(111) and D2O on Ni(111), revealing the distinct quantum nature in the two benchmark surface reactions. NE-RPMD successfully captures quantum tunneling in H2 dissociation at very low energies, where the quasi-classical trajectory (QCT) method suddenly fails. Additionally, QCT substantially overestimates S0 of D2O because of severe zero point energy (ZPE) leakage, even at collision energies greater than the ZPE-corrected barrier. Instead, NE-RPMD predicts S0 values of D2O in much improved agreement with reference results obtained by the quantum wavepacket method with reasonable corrections of the thermal contribution. Our results suggest NE-RPMD as a promising approach to model quantum effects in gas-surface reactions.

Dynamics of H + HeH+(v = 0, j = 0) → H2+ + He: Insight on the Possible Complex-Forming Behavior of the Reaction

The H + HeH+→ He + H2+ reaction has been studied by means of a combination of theoretical approaches: a statistical quantum method (SQM), ring polymer molecular dynamics (RPMD), and the quasiclassical trajectory (QCT) method. Cross sections and rate constants have been calculated in an attempt to investigate the dynamics of the process. The comparison with previous calculations and experimental results reveals that despite the fact that statistical predictions seem to reproduce some of the overall observed features, the analysis at a more detailed state-to-state level shows noticeable deviations from a complex-forming dynamics. We find some differences in cross sections and rate constants obtained in the QCT calculation with a Gaussian binning procedure with respect to previous works in which the standard histogram binning was employed.

HCS(A2A″)-based insights into the effect of vibrational excitation on the reactions C+SH (v = 0–20, j = 0) → S+CH, H+CS

The effect of vibrational excitation on reaction C+SH (v = 0–20, j = 0) → S+CH, H+CS is investigated on the excited potential energy surface of HCS(A2A″) by the quasi-classical trajectory method. The obtained reaction probability, total integral cross section (ICS), and the impact parameter show that the influence of vibration excitation presents different characteristics on different reaction channels. The vibrational state-resolved ICSs, differential cross sections as well as two-angle distribution functions P(θr), P(ϕr) of products for different vibrational quantum numbers of reactant are investigated. These results show that (i) the products have obvious forward–backward scattering feature; (ii) for different reactions, the distribution P(θr) varies with vibrational quantum number of reactant; (iii) at high vibrational excitations of the reactant, the insertion mechanism becomes apparent in this reaction, so the product molecules are more positively oriented along the positive direction of the scattering plane.

Thermal rate coefficients and kinetic isotope effects of the reaction HO + H2O → H2O + OH

Hydrogen-transfer reactions take place in a wide range of chemically active environments. In this work, thermal rate coefficients of the prototypical hydrogen-transfer reaction HO + H2O → H2O + OH and its various isotopologues are computed using both tunneling-corrected transition state and quasi-classical trajectory methods on a recently developed global potential energy surface. On the one hand, the calculated rate coefficients and kinetic isotope effects agree well with available experimental results, indicating the high fidelity of the potential energy surface. On the other hand, the observed normal primary and inverse secondary kinetic isotope effects appear to be prevalent in hydrogen abstraction reactions, which are rationalized by the change of classical and adiabatic minimum energy paths. In addition, there exists strong non-Arrhenius behavior at low temperatures due to the significant quantum tunneling effect.

Accurate global adiabatic potential energy surface for the ground state of AlH2+ by extrapolation to the complete basis set limit

ABSTRACT has very important research significance as the intermediate product of free radical . However, in Bell and Simons's [J. Phys. Chem. A. 103, 539–549 (1999)] study about , the maximum value of the integral cross section is about twice as large as the experimental value, which is caused by the lack of a precise global potential energy surface. An accurate full three-dimensional global potential energy surface for the electronic ground state of is reported in this paper by fitting a large amount of ab initio energies extrapolated to the complete basis set limit, which calculated based on aug-cc-pV(Q,5)Z basis sets and the multi-reference configuration interaction method with Davidson correction. The main geomorphic features of the novel potential energy surface are carefully investigated and have a great agreement with previous works. For examining the correctness of the new potential energy surface, based on the reaction , the integral cross section is calculated by employing the time-dependent wave packet theory and quasi-classical trajectory method. The dynamic results of two methods are in good agreement with the experimental values, which indicates the novel potential energy surface perfectly solves the divergence between the theoretical and the experimental value.

The Intricate Dynamics of the Si(3P) + OH(X2Π) Reaction.

The dynamics of the Si(3P) + OH(X2Π) → SiO(X1Σ+,v',j') + H(2S) reaction is investigated by means of the quasi-classical trajectory method on the electronic ground state X2A' potential energy surface in the 10-2-1 eV collision energy range. Although the reaction involves the formation of a long-lived intermediate complex, a high probability for back-dissociation to the reactants is found because of inefficient intravibrational redistribution of energy among the complex modes. At low collision energies, the reactive events are governed by a dynamics with mixed direct/indirect features. As the collision energy increases, the intermediate complex lifetime increases and final state distributions are found to be in reasonable agreement with statistical predictions obtained using the mean potential phase space theory, thus highlighting the indirect character of the process. These rich and puzzling dynamical features are in line with what has been previously observed for the S(3P) + OH(X2Π) reaction.