  The Kubo formula describes a current as a response to an external field. In
the case of heat conduction there is no such external field. We analyze why and
to what extend it is nevertheless justified to describe heat conduction in
modular quantum systems by the Kubo formula. Modular we call systems that may
be described as consisting of weakly coupled identical subsystems. We explain
in what sense this description applies to a large class of systems.
Furthermore, we numerically evaluate the Kubo formula for some finite modular
systems. We compare the results with data obtained from the direct numerical
solution of the corresponding time-dependent Schroedinger equation.
