Molecular Dynamics (MD) simulations are a necessary tool for interpreting experimental results and engineering new materials with desired properties. Compromises between the size of simulations and its accuracy are usually necessary. Classical simulations can explore systems of the size of million of atoms for microseconds, but are unable to describe bond formation/cleavage and the accuracy is intrinsically limited by the functional form of the Force Field used. Quantum Mechanical (QM) simulations can be extremely accurate, but they are tremendously limited in size and time.

Interpolation methods on ab-initio data, as the ones based on Reproducing Kernel Hilbert Space (RKHS)[1], can be implemented in classical MD packages. At a price of a small overhead respect to classical simulations, the wise choice of kernels are able to reproduce quantum-mechanical results for the interested properties. The one and three dimensional models, based on a previous work in our group [2] , are respectively in release and development in CHARMM [3].

The mono-dimensional case is successfully used for describing 2O -> O₂ recombination, on top and inside of a grain of amorphous solid water at 50 K [4]. The high rate of reactive events (~80% over 1000 simulations of 100 ps) confirms that the oxygen atoms have sufficient energy for overcoming the average encountered diffusional barriers (0.6 kcal/mol over a 0.1 kcal/mol temperature) [5] . The scarce coupling between O₂ vibration and the other modes can explain the reduced number of observations of O₂ gas in space, that will be trapped in the grain and possibly reacts with other incoming species.

The inclusion of multiple Potential Energy Surfaces via different kernels allows the study and implementation of non adiabatic effects during  simulations. Recombination on O₂ is performed, including Spin Orbit Coupling effects  between the ¹Σ_g excited  state and ³Σ_g ground state. Preliminary results show that the ¹Σ_g state is the favourite formation channel (66% of 1800  reactive events).

The three dimensional case is tested for the CO + O -> CO₂ formation in the same environmental conditions used for O₂. Compared to the previous simulations, a better coupling between the molecular modes and the water is observed, that has as a consequence a more efficient relaxation mechanism and higher diffusivity for the recombined molecule.

This study shows that RKHS is a powerful tool that allows the inclusion of reactive events and enhance the accuracy of calculations using classical MD packages.

[1] Timothy Hollebeek, Tak-San Ho, Herschel Rabitz, Annual Review of Physical Chemistry, 1999, 50,  537-570.
[2] Oliver T. Unke, Markus Meuwly, Journal of Chemical Information and Modelling, 2017, 57, 1923-1931.
[3] Bernard Brooks et al, Journal of Computational Chemistry, 2009, 30 ,1545-1614.
[4] Marco Pezzella, Markus Meuwly, Physisical Chemistry Chemical Physics, 2019, 21, 6247-6255.
[5] Marco Pezzella, Oliver T. Unke,  Markus Meuwly, Journal of Physical Chemistry Letters, 2018, 9, 1822-1826.