Ab initio calculations for high-pressure phases of Ar(H(2))(2).

High-pressure structures of Ar(H(2))(2) are studied with possible molecular dissociation and metallization using density functional theory in the local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange-correlation energy. We have estimated the transition pressure from the low-pressure MgZn(2)-type structure to the high-pressure AlB(2)-type structure to be around 240 GPa. The band gap of the MgZn(2)-type structure does not close, while the band gap of the AlB(2) closes at 420 GPa, which is much higher than the pressure of the band gap closure in pure hydrogen obtained by the same GGA. The lattice vibrations in those structures are also studied. The frequencies of the vibronic motions show good agreement with experiments. The molecular dissociation of the hydrogen in Ar(H(2))(2) occurs at higher pressures than in pure hydrogen.