  The origin of both weak temperature dependence and packing fraction
dependence of $T^{1/4}\eta^{3/2}$ in the thermal conductivity of the simple
Lennard-Jones (LJ) liquid is explored. In order to discuss the relative
contributions from attractive or repulsive part of the interaction potential
separately, the thermal conductivity of a series of Weeks-Chandler-Anderson
(WCA) fluids is calculated by molecular dynamics simulations. The results show
that the repulsive part plays the main role in the heat conduction, while the
attractive part has no direct effect on the thermal conductivity for a given
packing fraction. By investigating WCA fluids with potentials of varying
softness, we explain the difference observed between the LJ liquids such as
argon and Coulombic liquids such as NaCl.
