  We study the structural stability and dynamical properties of Ar(H2)2 under
pressure using first-principles and ab initio molecular dynamics techniques. At
low temperatures, Ar(H2)2 is found to stabilize in the cubic C15 Laves
structure (MgCu2) and not in the hexagonal C14 Laves structure (MgZn2) as it
has been assumed previously. Based on enthalpy energy and phonon calculations,
we propose a temperature-induced MgCu2 -> MgZn2 phase transition that may
rationalize the existing discrepancies between the sets of Raman and infrared
vibron measurements. Our AIMD simulations suggest that the melting line of
Ar(H2)2 presents negative slope in the interval 60 < P < 110 GPa. We explain
the origin of this intriguing physical phenomenon in terms of decoupling of the
Ar and H2 degrees of freedom and effective thermal-like excitations arising
from coexisting liquid H2 and solid Ar phases.
