Test of mode coupling theory for a supercooled liquid of diatomic molecules. II.q-dependent orientational correlators

Stefan Kämmerer, Walter Kob, Rolf Schilling
1998 Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics  
Using molecular dynamics computer simulations we study the dynamics of a molecular liquid by means of a general class of time-dependent correlators 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-dependence of the static correlators S_ll'^m(q) strongly depend on l, l' and m. The time dependent correlators are calculated for l=l'. A thorough test of
more » ... predictions of mode coupling theory (MCT) is performed for S_ll^m(q,t) 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 temperature T_c, at which the dynamics changes significantly. The first scaling law of MCT, which involves the critical correlator G(t), holds for l>=2, but no critical law is observed. Since this is true for the same exponent parameter lambda as obtained for the TDOF, we obtain a consistent description of both, the TDOF and ODOF, with the exception of l=1. This different behavior for l 1 and l=1 can also be seen from the corresponding susceptibilities (chi")_ll^m(q,omega) which exhibit a minimum at about the same frequency omega_min for all q and all l 1, in contrast to (chi")_11^m(q,omega) for which omega'_min approx 10 omega_min . The asymptotic regime, for which the first scaling law holds, shrinks with increasing l. The second scaling law of MCT (time-temperature superposition principle) is reasonably fulfilled for l 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) C_l^(s)(t) F_s(q,t) for all m.
doi:10.1103/physreve.58.2141 fatcat:mggf4k2fwrfqjmg2zcgoi7gyau