Quasiclassical trajectory calculations for Li(22PJ) + H2 → LiH(X1Σ+) + H: Influence by vibrational excitation and translational energy

New Diabatic Potential Energy Surfaces for the Li + H2 Reaction and Time-Dependent Quantum Wave Packet Studies

New global diabatic potential energy surfaces (DPESs) for the ground (12A') and first excited (22A') states for the Li + H2 system were developed, with more than 30,000 energy points at the IC-MRCI+Q level of theory, utilizing the aug-cc-pV5Z basis set for the H atoms and the cc-pCV5Z basis set for the Li atom, fitted by a single neural network (NN) with symmetry. Product state-resolved quantum dynamics calculations of the nonadiabatic reaction Li (2P) + H2 (X 1 ∑g+, v0 = 0, j0 = 0) → LiH (X 1∑+) + H(2S) were carried out using these new DPESs and also the previous HYLC-DPESs. The numerical results suggested that our newly constructed DPESs provided an accurate description of the LiH2 system.

Non-adiabatic dynamics studies of the H(2S) + LiH(X1Σ+) reaction by time-dependent wave packet method

Non-adiabatic dynamics studies of the H + LiH (v₀ = 0, j₀ = 0) reaction are carried out based on the diabatic potential energy surfaces (PESs) reported by He and co-workers (Sci. Rep., 2016, 6, 25083) in the collision energy range from 0.001 to 1.0 eV. The H(²S) + LiH(X¹Σ⁺) → Li(2²S)/Li(2²P) + H₂(X¹Σ_g⁺) depletion reactions and the H'(²S) + LiH(X¹Σ⁺) → H(²S) + LiH'(X¹Σ⁺) exchange reaction are studied at the state-to-state level of theory. The dynamics properties, such as reaction probability, integral cross section, differential cross section, the ro-vibrational state distributions of the product and specific-state rate constant are calculated. In addition, the dynamics values of the H(²S) + LiH(X¹Σ⁺) → Li(2²S) + H₂(X¹Σ_g⁺) depletion reaction are compared with available theoretical results, which are based on the adiabatic PESs. Large discrepancies are found between the adiabatic and diabatic values, especially at low J values, which indicate that the non-adiabatic effect is very great and cannot be simply ignored. Furthermore, the deviations between adiabatic and diabatic values gradually decrease as the collision energy increases.

Accurate potential energy surfaces for the first two lowest electronic states of the Li (2p) + H2 reaction

The accuracy of three-dimensional adiabatic and diabatic potential energy surfaces is calculated using ab initio methods and is numerically fitted for the two lowest electronic states 1 and 2²A′ of the LiH₂ system, which are very important for the Li (2p) + H₂ reaction. The finite difference method is performed to generate the mixing angles, which are used to educe the diabatic potential from the adiabatic potential. The accurate conical intersection (CI) is studied in this work with three different basis sets. The energy of the conical intersection is slightly lower (nearly 0.12 eV) than that of the perpendicular intermediate on the first excited state. By analyzing the potential energy surfaces in this work we can suggest that the most possible reaction pathway for the title reaction is Li (2p) + H₂ → LiH₂ (2²A′) (C_2v) → CI → LiH₂ (1²A′) (C_2v) → LiH⋯H → LiH (X¹∑_g⁺) + H. The conical intersection and (2²A′) intermediate may play a vital role in the title reaction.

Accurate diabatic potential energy surfaces for the Li (2p) + H₂ → LiH (X) + H reaction are produced.

Influence of rovibrational excitation on the non-diabatic state-to-state dynamics for the Li(2p) + H2 → LiH + H reaction

The non-adiabatic state-to-state dynamics of the Li(2p) + H2 → LiH + H reaction has been studied using the time-dependent wave packet method, based on a set of diabatic potential energy surfaces recently developed by our group. Integral cross sections (ICSs) can be increase more than an order of magnitude by the vibrational excitation of H2, whereas the ICSs are barely affected by the rotational excitation of H2. Moreover, ICSs of the title reaction with vibrationally excited H2 decrease rapidly with increasing collision energy, which is a typical feature of non-threshold reaction. This phenomenon implies that the title reaction can transformed from an endothermic to an exothermic reaction by vibrational excitation of H2. With the increase of the collision energy, the sideways and backward scattered tendencies of LiH for the Li(2p) + H2(v = 0, j = 0, 1) → LiH + H reactions are enhanced slightly, while the backward scattering tendency of LiH for the Li(2p) + H2(v = 1, j = 0) → LiH + H reaction becomes remarkably weakened. For the reaction with vibrationally excited H2 molecule, both direct and indirect reaction mechanism exist simultaneously.

Global diabatic potential energy surfaces and quantum dynamical studies for the Li(2p) + H2(X(1)Σ(+)g) → LiH(X(1)Σ(+)) + H reaction

The global diabatic potential energy surfaces which are correlated with the ground state 1A' and the excited state 2A' of the Li(2p) + H2 reaction are presented in this study. The multi-reference configuration interaction method and large basis sets (aug-cc-pVQZ for H atom and cc-pwCVQZ for Li atom) were employed in the ab initio single-point energy calculations. The diabatic potential energies were generated by the diabatization scheme based on transition dipole moment operators. The neural network method was utilized to fit the matrix elements of the diabatic energy surfaces, and the root mean square errors were extremely small (3.69 meV for , 5.34 meV for and 5.06 meV for ). The topographical features of the diabatic potential energy surfaces were characterized and the surfaces were found to be sufficiently smooth for the dynamical calculation. The crossing seam of the conical intersections between the and surfaces were pinpointed. Based on this new analytical diabatic potential energy surfaces, time-dependent wave packet calculation were conducted to investigate the mechanism of the title reaction. At low collision energies, the product LiH molecule tends to forward scattering, while at high collision energies, the forward and backward scatterings exist simultaneously.

A new potential energy surface for the ground electronic state of the LiH2 system, and dynamics studies on the H((2)S) + LiH(X(1)Σ(+)) → Li((2)S) + H2(X(1)Σg(+)) reaction

A new global potential energy surface (PES) is obtained for the ground electronic state of the LiH2 system based on high-level energies. The energy points are calculated at the multireference configuration interaction level with aug-cc-pVXZ (X = Q, 5) basis sets, and these energies are extrapolated to the complete basis set limit. The neural network method and hierarchical construction scheme are applied in the fitting process and the root mean square error of the fitting result is very small (0.004 eV). The dissociation energies and equilibrium distances for LiH(X(1)Σ(+)) and H2(X(1)Σg(+)) obtained from the new PES are in good agreement with the experimental data. On the new PES, time-dependent wave packet studies for the H((2)S) + LiH(X(1)Σ(+)) → Li((2)S) + H2(X(1)Σg(+)) reaction have been carried out. In this reaction, no threshold is found due to the absence of an energy barrier on the minimum energy path. The calculated integral cross sections are high at low collision energy and will decrease with the increase of the collision energy. The product molecule H2 tends to be forward scattering due to direct reactive collisions, which becomes more evident at higher collision energies.

QUASI-CLASSICAL TRAJECTORY STUDY OF THE REACTION PROBABILITY AND CROSS SECTION OF THE REACTION LiH + H

Based on the new accurate potential energy surface of the ground state of LiH2 system. Quasi-classical trajectory (QCT) calculations were carried out for the reaction LiH + H . The reaction probability of the title reaction for J = 0 has been calculated. The reaction cross sections were calculated as functions of the collision energy in the range 0.1–2.5 eV. The results were found to be well consistent with the previous real wave packet (RWP) and QCT results.