systems at mesoscopic scales [@Nyquist1928; @OM1953; @MO1953]. Its generalization to nonequilibrium steady states, though, is still actively developed, as suggested by a number of recent publications [@KSSH2015I; @KSSH2015II; @MorgadoQ2016; @Queiros2016; @PRE2016; @PRE2016II; @PRE2016III]. A common objective of these studies is to provide a statistical account of an externally applied force together with its spontaneous variations, which manifest themselves in fluctuations of the resulting conjugate current in a system of interest.

In our most recent paper [@PRE2016III] we showed, that a second-order Langevin equation, suggested in Ref. [@MO1953], provides excellent means for quantitative studies of fluctuations at mesoscopic scales in both, equilibrium and nonequilibrium, steady-state systems. In particular, it yields an accurate analytical model of the time autocorrelation function for currents, which was successfully applied to evaluate the Green-Kubo formula for a transport coefficient. For a general fluctuating quantity $\alpha(t)$, the Langevin equation of the second order in time, $t$, reads: $${\label{eq:lgv1}}
    \ddot\alpha(t) + a \dot\alpha(t) + b^2 \alpha(t) = r(t),$$ where $a > 0$, $b > 0$ are constants, while $r(t)$ is a random noise.

In the macroscopic limit, [Eq. (\[eq:lgv1\])]{} transforms into a deterministic equation [@OM1953 Sec. 2-3] and, therefore, $r(t)$ must then in general become a constant, $f 