Path-integral Monte Carlo simulation of the recombination of two Al atoms embedded in parahydrogen

We report the use of path-integral Monte Carlo (PIMC) simulations in the study of the stability against recombination of two Al atoms trapped in solid parahydrogen (pH2) at 4 K. The many-body interactions involving open-shell Al atoms are described with a pairwise additive Hamiltonian model. To estimate the lifetime against recombination, we use PIMC simulations to define an effective potential averaged over the position of the pH2 molecules, followed by a transition-state treatment. Different initial embedding sites are explored. If the initial substitution sites are within a distance of approximately 13 bohrs, the Al atoms will significantly distort the lattice structure to allow recombination, with an accompanying release of energy during the process. For substitution distances longer than approximately 14 bohrs, the dispersion of Al atoms is shown to be metastable, with lifetimes varying from approximately 30 min to several days. The electronic anisotropy is a factor that helps to stabilize the dispersion.