Representations of the three lowest 2 A′ adiabatic electronic potential energy surfaces for the ArH 2 + reactive system with machine learning Maarten Konings , J. N. Harvey and J. Loreau KU Leuven, Division of Quantum Chemistry and Physical Chemistry, Belgium The argonium molecular cation, ArH + , can be formed in space through the exothermic reactions,Ar ( 1 S ) + H 2 + ( X 2 Σ + g )→ ArH + ( X 1 Σ + ) + H ( 2 S ), and Ar + ( 2 P 3/2,1/2 ) + H 2 ( X 1 Σ + g )→ ArH + ( X 1 Σ + ) + H ( 2 S ).The adiabatic electronic potential energy surface (PES) for the first reaction is the only reactive one, while the second reaction relies on the strong electronic coupling in the entrance arrangement, giving rise to non-adiabatic transitions. Non-adiabatic reaction dynamics studies of these reactions have been reported over the past decades, usually based on DIMZO (diatomics in molecule with zero overlap) PESs [1-6]. Here, we present, to the best of our knowledge, the first accurate full-dimensional representations of the lowest coupled 2 A’ adiabatic PESs, with inclusion of spin-orbit coupling, obtained by means of machine learning (ML) methods [8]. In particular, the technique of gaussian process (GP) regression was used, assuming the functional form of the kernels, k( x i , x j ), in the covariance matrix to be of the form, k( x i , x j )= D ( x i , x j ,σ) M ν ( x i , x j , l )+ W ( x i , x j ,n)[7], where x i , x j are two geometries of ArH 2 + , and D , M ν and W are the dot product, Matérn and white noise kernels, respectively. The necessary ab initio computations were performed at the ic-MRCI+Q/aug-cc-pVQZ level of theory. The dynamics of the formation of ArH + is currently being studied, both electronically adiabatic by means of a time-independent quantum scattering approach, and electronically non-adiabatic by means of the quasi-classical trajectory method with surface hopping, with the goal of obtaining state-to-state reaction rate coefficients.
Figure 1: All three diabatic/adiabatic 2 A’ PESs in the entrance arrangement (left), and a contour plot based on the ML representation of the ground state adiabatic PES for one of the linear geometries of the ArH 2 + system (right). References
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4. M. Baer and H. Nakamura, The Journal of Chemical Physics 87 , 4651–4664 (1987). 5. F. Aguillon and M. Sizun, The Journal of Chemical Physics 106 , 9551– 9562 (1997). 6. [M. Sizun, J.-B. Song, and E. A. Gislason, The Journal of Chemical Physics 116 , 2888–2895 (2002). 7. A. Christianen, T. Karman, R. A. Vargas-Hernández, G. C. Groenenboom, and R. V. Krems, The Journal of Chemical Physics 150 , 064106 (2019). 8. M. Konings, J. N. Harvey, J. Loreau, paper in preparation (2023).
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