TEST OF MODE-COUPLING THEORY FOR A SUPERCOOLED LIQUID OF DIATOMIC-MOLECULES - II - Q-DEPENDENT ORIENTATIONAL CORRELATORS

Using molecular-dynamics computer simulations, we study the dynamics o f a molecular liquid by means of a general class of time-dependent cor relators S-ll(m),(q,t), which explicitly involve translational (TDOF) and orientational degrees of freedom (ODOF). The system is composed of rigid, linear molecules with Lennard-Jones interactions. The q depend ence of the static correlators S-ll(m)(q) strongly depends on l, l', a nd m. The time-dependent correlators are calculated for l = l'. A test of some of the predictions of mode coupling theory (MCT) is performed for S-ll(m)(q,t) for l = 1,2 and its self-part S(ll)((s)m)(q,t), for l = 1,...,6. We find a clear signature for the existence of a single t emperature T-c, at which the nature of the dynamics changes significan tly. In the first scaling law regime of MCT it is found that the vario us correlators can be fitted with the beta correlator G(t), with the e xception of those with l = 1. Since this is true for the same exponent parameter lambda as obtained for the TDOF, we thus find that MCT give s a consistent description of the dynamics of the TDOF as well as the one of the ODOF, with the exception of l = 1. This different behavior for l not equal 1 and l = 1 can also be seen from the corresponding su sceptibilities (chi '')(ll)(m)(q,omega), which exhibit a minimum at ab out the same frequency omega(min), for all q and all l not equal 1, in contrast to (chi '')(11)(m)(q,omega) for which omega(min)' approximat e to 10 omega(min). The asymptotic regime, for which the first scaling law holds, shrinks with increasing 1. The second scaling law of MCT ( time-temperature superposition principle) is reasonably fulfilled for l not equal 1 but not for l = 1. Furthermore, we show that the q and ( l,m) dependence of the self-part approximately factorizes, i.e., S(ll) ((s)m)(q, t) congruent to C-l((s))(t)F-s(q, t) for all m.