In fluid transport across nanopores, there is a fundamental dissipation that
arises from the connection between the pore and the macroscopic reservoirs.
This entrance effect can hinder the whole transport in certain situations, for
short pores and/or highly slipping channels. In this paper, we explore the
hydrodynamic permeability of hourglass shape nanopores using molecular dynamics
(MD) simulations, with the central pore size ranging from several nanometers
down to a few Angstr{\"o}ms. Surprisingly, we find a very good agreement
between MD results and continuum hydrodynamic predictions, even for the
smallest systems undergoing single file transport of water. An optimum of
permeability is found for an opening angle around 5 degree, in agreement with
continuum predictions, yielding a permeability five times larger than for a
straight nanotube. Moreover, we find that the permeability of hourglass shape
nanopores is even larger than single nanopores pierced in a molecular thin
graphene sheet. This suggests that designing the geometry of nanopores may help
considerably increasing the macroscopic permeability of membranes
