  We re-derive the expression for the heat current for a classical system
subject to periodic boundary conditions and show that it can be written as a
sum of two terms. The first term is a time derivative of the first moment of
the system energy density while the second term is expressed through the energy
transfer rate through the periodic boundary. We show that in solids the second
term alone leads to the same thermal conductivity as the full expression for
the heat current when used in the Green-Kubo approach. More generally, energy
passing though any surface formed by translation of the original periodic
boundary can be used to calculate thermal conductivity. These statements are
verified for two systems: crystalline argon and crystals of argon and krypton
forming an interface.
