Quantum Monte-Carlo Simulations of Protonated Argon Clusters*

In this work, we explore the ground state configurations of protonated argon clusters, Ar_nH^+, for n up to 60. In particular, to find the positions of the argon atoms and ion which minimizes the total potential energy (modeled by a pairwise Lennard Jones), we use basin hopping, which Metropolis-samples the coordinates in the space of potential function local minima. To ensure that the global minima are found, we run dozens of instances of basin hopping in parallel, with a temperature corresponding to the distance between successive local minima, and we feed an analytic expression for the gradient of the potential energy to the local minimizer, to speed up the algorithm. To account for the zero-point energy, we then run diffusion Monte Carlo (DMC), where all random walkers start with the basin hopping configuration obtained above. 

We find that the most stable geometries exhibit different symmetries as each additional atom is added. The Ar_nH^+ clusters with the highest binding energies calculated from DMC are the most stable clusters in principle, and these clusters are found to mostly agree with the experimentally highest-yielded clusters in mass spectrometry by Gatchel et al (2018) [1]. Moving forward, to obtain a more complete description, we will look into the dynamics of the nucleation of these protonated argon clusters. 

[1] Michael Gatchell et al. (2018). Magic sizes of cationic and protonated argon clusters. Physical Review A.