  The thermal conductivity of nanometric objects or nanostructured materials
can be determined using non equilibrium molecular dynamics (NEMD) simulations.
The technique is simple in its principle, and resembles a numerical guarded hot
plate experiment. The 'sample' is placed between a hot source and a cold source
consisting of thermostatted sets of atoms. The thermal conductivity is obtained
from the heat flux crossing the sample and the temperature profile in the
system. Simulations results, however, exhibit a strong dependence of the
thermal conductivity on the sample size. In this paper, we discuss the physical
origin of this size dependence, by comparing MD results with those obtained
from simple models of thermal conductivity based on harmonic theory of solids.
A model is proposed to explain the variation of the thermal conductivity with
system size.
