Thermal transport through nanosystems is central to numerous processes in chemistry, material sciences, and electrical and mechanical engineering, with classical molecular dynamics as the key simulation tool. Here, we focus on thermal junctions with a molecule bridging two solids that are maintained at different temperatures. The classical steady state heat current in this system can be simulated in different ways, either at the interfaces with the solids, which are represented by thermostats, or between atoms within the conducting molecule. We show that while the latter, intramolecular definition feasibly converges to the correct limit, the molecule–thermostat interface definition is more challenging to converge to the correct result. The problem with the interface definition is demonstrated by simulating heat transport in harmonic and anharmonic one-dimensional chains illustrating unphysical effects such as thermal rectification in harmonic junctions.

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We study universal aspects of fluctuations in an ensemble of noninteracting continuous quantum thermal machines in the steady state limit. Considering an individual machine, such as a refrigerator, in which relative fluctuations (and high order cumulants) of the cooling heat current to the absorbed heat current, $\eta^{(n)}$, are upper-bounded, $\eta^{(n)}\leq \eta_C^n$ with $n\geq 2$ and $\eta_C$ the Carnot efficiency, we prove that an {\it ensemble} of $N$ distinct machines similarly satisfies this upper bound on the relative fluctuations of the ensemble, $\eta_N^{(n)}\leq \eta_C^n$. For an ensemble of distinct quantum {\it refrigerators} with components operating in the tight coupling limit we further prove the existence of a {\it lower bound} on $\eta_N^{(n)}$ in specific cases, exemplified on three-level quantum absorption refrigerators and resonant-energy thermoelectric junctions. Beyond special cases, the existence of a lower bound on $\eta_N^{(2)}$ for an ensemble of quantum refrigerators is demonstrated by numerical simulations.

Understanding excitation and charge transfer in disordered media is a significant challenge in chemistry, biophysics and material science. We study two experimentally-relevant measures for carriers transfer in finite-size chains, the trapping mean first-passage time (MFPT) and the steady state transfer time (SSTT). We discuss the relationship between these measures, and derive analytic formulae for one-dimensional chains. We exemplify the behavior of these timescales in different motifs: donor-bridge-acceptor systems, biased chains, and alternating and stacked co-polymers. We find that the MFPT and the SSTT may administer different, complementary information on the system, jointly reporting on molecular length and energetics. Under constraints such as fixed donoracceptor energy bias, we show that the MFPT and the SSTT are optimized (minimized) under fundamentally different internal potential profiles. This study brings insights on the behavior of the MFPT and the SSTT, and suggests that it is beneficial to perform both transient and steady state measurements on a conducing network so as to gather a more complete picture of its properties. arXiv:1809.06281v1 [cond-mat.mes-hall] 17 Sep 2018 = e −(Ei+1−Ei)/T ; we set the Boltzmann constant as k B ≡ 1. Nevertheless, our general results in this Section do not rely on this particular choice for the rates.

Thermal transport through nanosystems is central to numerous processes in chemistry, material sciences, electrical and mechanical engineering, with classical molecular dynamics as the key simulation tool. Here we focus on thermal junctions with a molecule bridging two solids that are maintained at different temperatures. The classical steady state heat current in this system can be simulated in different ways, either at the interfaces with the solids, which are represented by thermostats, or between atoms within the conducting molecule. We show that while the latter, intramolecular definition feasibly converges to the correct limit, the molecule-thermostat interface definition is more challenging to converge to the correct result. The problem with the interface definition is demonstrated by simulating heat transport in harmonic and anharmonic one-dimensional chains illustrating unphysical effects such as thermal rectification in harmonic junctions.