  Thermal transport coefficients are independent of the specific microscopic
expression for the energy density and current from which they can be derived
through the Green-Kubo formula. We discuss this independence in terms of a kind
of gauge invariance resulting from energy conservation and extensivity, and
demonstrate it numerically for a Lennard-Jones fluid, where different forms of
the microscopic energy density lead to different time correlation functions for
the heat flux, all of them, however, resulting in the same value for the
thermal conductivity.
