  The thermal conductivity of classical multi-component fluids is seemingly
affected by the intrinsic arbitrariness in the definition of the atomic
energies and it is ill-conditioned numerically, when evaluated from the
Green-Kubo theory of linear response. To cope with these two problems we
introduce two new concepts: a convective invariance principle for transport
coefficients, in the first case, and multi-variate cepstral analysis, in the
second. A combination of these two concepts allows one to substantially reduce
the noise affecting the estimate of the thermal conductivity from equilibrium
molecular dynamics, even for one-component systems.
