Computing nonequilibrium transport from short-time transients: From Lorentz gas to heat conduction in one-dimensional...

We test the Transient Time Correlation Function (TTCF) method to compute nonequilibrium transport coefficients, highlighting its conceptual and practical differences from the standard time-average approach. While time averages extract transport properties from long stationary trajectories and discard transient dynamics, TTCF adopts the complementary strategy: it exploits the information contained in short-time transients following the onset of an external perturbation while discarding the long-time evolution once stationarity is reached. We revisit the theoretical framework of TTCF and assess its numerical performance through representative case studies: the Lorentz gas and a many-body system, namely, a chain of oscillators with an anharmonic pinning potential. By direct comparison with time averages, we show that for the Lorentz gas, TTCF yields consistent transport coefficients in both linear and nonlinear regimes at a reduced computational cost. Moreover, TTCF displays superior precision in the linear-response regime and remains reliable in nonergodic situations, revealing the presence of regions of phase space corresponding to different behaviors, as well as the possibility of phase transitions. For the anharmonic chain, we show that TTCF is a scalable and efficient alternative for the numerical study of nonequilibrium transport.