A microscopic formalism to calculate thermal transport coefficients is presented based on a thermal vector potential, whose time-derivative is related to a thermal force. The formalism is free from unphysical divergences reported to arise when Luttinger's formalism is applied naively, because the equilibrium (\textquoteleft diamagnetic\textquoteright) currents are treated consistently. The mathematical structure for thermal transport coefficients are shown to be identical with the electric ones

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... if the electric charge is replaced by energy. The results indicates that the thermal vector potential couples to energy current via the minimal coupling.