  Parity preserving relativistic fluids in four spacetime dimensions admit
seven independent thermodynamic susceptibilities at the second order in the
hydrodynamic derivative expansion. We compute all parity-even second order
thermodynamic susceptibilities for a free massive Dirac field at zero
temperature and a nonzero chemical potential, based on the Kubo formulas
reported in P. Kovtun and A. Shukla, Kubo formulas for thermodynamic transport
coefficients, J. High Energy Phys. 10 (2018) 007. We also compute the second
order constitutive relations for the energy-momentum tensor and the conserved
current in the absence of external gauge fields.
