  We demonstrate that the proper calculation of the linear response for
finite-size systems can only be performed if the coupling to the leads/baths is
explicitly taken into consideration. We exemplify this by obtaining a Kubo-type
formula for heat transport in a finite-size system coupled to two thermal
baths, kept at different temperatures. We show that the proper calculation
results in a well-behaved response, without the singular contributions from
degenerate states encountered when Kubo formulae for infinite-size systems are
inappropriately used for finite-size systems.
