  The nonequilibrium steady states of quantum materials have many challenges.
Here, we highlight issues with the relaxation time approximation (RTA) for the
DC conductivity in insulating systems. The RTA to the quantum master equation
(QME) is frequently employed as a simple method, yet this phenomenological
approach is exposed as a fatal approximation, displaying metallic DC
conductivity in insulating systems within the linear response regime. We find
that the unexpected metallic behavior is caused by the fact that the density
matrix in the RTA incompletely incorporates the first order of the external
field. To solve this problem, we have derived a new calculation scheme based on
the QME that ensure correct behavior in low electric fields. Our method
reproduces well the overall features of the exact electric currents in the
whole field region. It is not time-consuming, and its application to lattice
systems is straightforward. This method will encourage progress in this
research area as a simple way to more accurately describe nonequilibrium steady
states.
