  The strengths and short-comings of the point-dipole model for polar fluids of
spherical molecules are illustrated by considering the physically more relevant
case of extended dipoles formed by two opposite charges $\pm q$ separated by a
distance $d$ (dipole moment $\mu=q d$). Extensive Molecular Dynamics
simulations on a high density dipolar fluid are used to analyse the dependence
of the pair structure, dielectric constant $\eps$ and dynamics as a function of
the ratio $d/\sigma$ ($\sig$ is the molecular diameter), for a fixed dipole
moment $\mu$. The point dipole model is found to agree well with the extended
dipole model up to $d/\sig \simeq 0.3$. Beyond that ratio, $\eps$ shows a
non-trivial variation with $d/\sig$. When $d/\sig>0.6$, a transition is
observed towards a hexagonal columnar phase; the corresponding value of the
dipole moment, $\mu^2/\sig^3 k T=3$, is found to be substantially lower than
the value of the point dipole required to drive a similar transition.
