  A fundamental result in the theory of Brownian motion is the
Einstein-Sutherland relation between mobility and diffusion constant. Any
classical linear response transport coefficient obeys a similar
Einstein-Helfand relation. We show in this work how to derive the interleaflet
friction coefficient of lipid bilayer by means of an adequate generalisation of
the Einstein relation. Special attention must be paid in practical cases to the
constraints on the system center of mass position that must be enforced when
coupling the system to thermostat.
