Melting Under Extreme Conditions: Ab Initio Monte Carlo Simulations

Melting results for rare gas clusters and solids under extreme pressures and magnetic fields are presented. Phase transitions are simulated by exploring phase space with classical (parallel-tempering) Monte Carlo methods combined with a very accurate computation of the interaction energy of the sampled configurations. We employ many-body expansions where the total interaction energy of the N-atom system is obtained by decomposing the total energy into two-, three- and higher-body fragments.

For rare gases, this approach works well for melting under ambient conditions¹ and for argon for pressures up to 100 GPa, 1 Million times the atmospheric pressure². We work in the isobaric, isothermal ensemble, allowing for atom displacements as well as cell volume adjustments employing periodic boundary conditions. Excellent agreement with experimental data is found up to very high pressures.

Neon clusters in high magnetic fields show elevated melting points and also give fascinating insight in the melting process itself.
In a strong homogeneous magnetic field applied in z-direction the clusters get squeezed in the perpendicular plane as also observed for atoms. Around the melting temperature, we observe an additional squeezing parallel to the magnetic field due to perpendicular paramagnetic bonding responsible for the enhanced melting points. For these studies, dimer potential curves were computed on MP2 and coupled-cluster level depending on the magnetic field, the interatomic distance and the angle of the molecular axis with the magnetic field direction.

1 O.Smits, P.Jerabek, E.Pahl, P.Schwerdtfeger, Angew. Chem. Int. Ed. (2018) 57, 9961 and references therein.
2 J.Wiebke, E.Pahl, and P.Schwerdtfeger, Angew. Chem. Int. Ed. (2013) 52, 13202