The temperature dependent dielectric function of liquid benzene: Interpretation of THz spectroscopy data by molecular dynamics simulation

The dielectric function of liquid benzene at -4 degreesC, 21 degreesC, and 65 degreesC has been calculated from molecular dynamics simulations. The si mulated dielectric loss curves reproduce the experimental temperature and d ensity trends. In order to investigate the detailed influence of temperatur e and density changes as well as the underlying molecular mechanism we have taken advantage of different partitioning and projection schemes for the t otal dipole moment time correlation function (TCF). The study of the n-body partitioning showed that the temperature dependence of the two- and three- body contributions at t=0 can be explained solely by density change argumen ts. The molecular projection scheme showed that the dielectric loss is gove rned by out-of-plane libration at all temperatures. In-plane libration was found to contribute significantly only below 2 THz. Below 1 THz, diffusion, manifested as the negative cross correlation between the out-of-plane and the in-plane TCF's, plays a role very different from that of the directly o bserved diffusion in dipolar liquids. It has further been established that it is highly problematic to carry out an analysis of the dielectric loss fu nction in terms of the molecular axis rotational TCF's which is a common pr ocedure for the absorption spectrum. This problem was, however, solved by e mploying a molecular projection scheme. (C) 2001 American Institute of Phys ics.