Globally Accurate Gaussian Process Potential Energy Surface and Quantum Dynamics Studies on the Li(2S) + Na2 → LiNa + Na Reaction at Low Collision Energies

Abstract

The LiNa₂ reactive system has recently received great attention in the experimental study of ultracold chemical reactions, but the corresponding theoretical calculations have not been carried out. Here, we report the first globally accurate ground-state LiNa₂ potential energy surface (PES) using a Gaussian process model based on only 1776 actively selected high-level ab initio training points. The constructed PES had high precision and strong generalization capability. On the new PES, the quantum dynamics calculations on the Li(²S) + Na₂(v = 0, j = 0) → LiNa + Na reaction were carried out in the 0.001-0.01 eV collision energy range using an improved time-dependent wave packet method. The calculated results indicate that this reaction is dominated by a complex-forming mechanism at low collision energies. The presented dynamics data provide guidance for experimental research, and the newly constructed PES could be further used for ultracold reaction dynamics calculations on this reactive system.

Keywords: Gaussian process; ab initio; potential energy surface; reaction dynamics; time-dependent wave packet.

Figure 1

Highest absolute error (a) and RMSE (b) of the remaining test data as a function of the number of training points for the LiNa₂ PES modeled by the GP model based on the two active learning schemes.

Figure 2

Predictive error distributions in the test database (13,453 points) of the ground-state LiNa₂ PES constructed by the GP model with 1776 points.

Figure 3

Contour plots of the ground-state LiNa₂ PES at C_2v (a) and D_∞h (b) symmetries.

Figure 4

Three-dimensional diagrams and the corresponding contour maps of the ground-state LiNa₂ PES at fixed Li–Na–Na angles (45°, 90°, 135° and 180°).

Figure 5

MEPs at four Li–Na–Na angles (45°, 90°,135°, and 180°) and global MEP of the Li(²S) + Na₂ → LiNa + Na reaction calculated by the GP PES.

Figure 6

Long-range interaction potentials obtained on the GP PES along the radial coordinate at four fixed Jacobi angles (θ = 5°, 30°, 60° and 85°) with the bond length of Na₂ fixed at 6.01 a₀, in comparison with the original ab initio energies.

Figure 7

Collision energy dependence of total reaction probabilities with six partial waves (J = 0, 10, 20, 30, 40 and 50) of the Li(²S) + Na₂(v = 0, j = 0) → LiNa + Na reaction calculated by the improved TDWP method on the GP PES.

Figure 8

Rovibrationally state-resolved ICSs of the product LiNa molecules of the Li(²S) + Na₂(v₀ = 0, j₀ = 0) → LiNa + Na reaction at four collision energies (0.001, 0.004, 0.008 and 0.010 eV) calculated by the improved TDWP method on the GP PES.

Figure 9

Total DCSs of the product LiNa molecules of the Li(²S) + Na₂(v = 0, j = 0) → LiNa + Na reaction at four collision energies (0.001, 0.004, 0.008 and 0.010 eV) as a function of scattering angle calculated by the improved TDWP method on the GP PES.