Test of mode coupling theory for a supercooled liquid of diatomic molecules. I. Translational degrees of freedom

A molecular-dynamics simulation is performed for a supercooled liquid of rigid diatomic molecules. The time-dependent self and collective density correlators of the molecular centers of mass are determined and compared with the predictions of the ideal mode coupling theory (MCT) for simple liquids. This is done in real as well as in momentum space. One of the main results is the existence of a unique transition temperature T-c, at which the dynamics crosses over from an ergodic to a quasinonergodic behavior. The value for T-c agrees within the error bars with that found earlier for the orientational dynamics. In the first scaling law regime of MCT, also called the beta regime, we find that the correlators in the late stage of the beta regime can be fitted well by the von Schweidler law. Although we do not observe the critical decay predicted by MCT for the early beta-relaxation regime in its pure form, our relaxation curves suggest that this decay is indeed present. In this first scaling regime, a consistent description within ideal MCT emerges only, if the next order correction to the asymptotic law is taken into account. This correction is almost negligible for q = q(max), the position of the main peak in the static structure factor S(q), but becomes important for q = q(min), the position of its first minimum. The second scaling law, i.e., the time-temperature superposition principle, holds reasonably well for the self and collective density correlators and different values for q. The alpha-relaxation times tau(q)((s)) and tau(q) follow a power law in T - T-c over two to three decades. The corresponding exponent gamma is practically q independent and is around 2.55. This value is in agreement with the one predicted by MCT from the value of the von Schweidler exponent but at variance with the corresponding exponent gamma approximate to 1.6 obtained for the orientational correlators C-1((s))(t) and C-1(t), studied in a previous paper.