Neptune-sized planets dominate exoplanet discovery surveys. Their interior structure is thought to comprise hydrogen- and helium-rich atmospheres, followed by expansive 'hot ice' mantle regions and, possibly, small rocky cores. High-pressure/high-temperature studies of the majority components of the dense 'hot ice' layer (water, methane, and ammonia) have, individually, reached the conditions present at the lower mantle regions of such planets. Their mixtures, and their interactions with the other planetary layers (the atmosphere and core materials) are much less studied, probably due to the additional compositional complexity and associated challenges regarding rare events (in simulations) and observational uncertainties (in experiments). However, molecular mixtures allow chemical reactions that can drastically alter the composition at specific conditions and therefore govern the depth-density profile, thermal conductivity and evolution, interior convection and magnetism, etc.
In this talk I will discuss some of the recent computational progress in studies of complex molecular mixtures at extreme conditions, including 'hot ice' mixtures (such as ammonia-water), ices mixing with atmospheric materials (such as methane or ammonia in the presence of excess hydrogen or helium), and the fate of relatively uncommon species (such as hydrogen sulfide). Based on crystal structure predictions and first principles calculations we will develop a picture of the chemical bonding present in cold dense mixtures before mapping out the high-temperature phase diagrams through ab-initio molecular dynamics simulations and characterising the emerging states up to the fluid regime, juxtaposed against expected conditions in giant icy planets.