  An optimized molecular model for ammonia, which is based on a previous work
of Kristoef et al., Mol. Phys. 97 (1999) 1129--1137, is presented. Improvements
are achieved by including data on geometry and electrostatics from quantum
mechanical calculations in a first model. Afterwards the parameters of the
Lennard-Jones potential, modeling dispersive and repulsive interactions, are
optimized to experimental vapor-liquid equilibrium data of pure ammonia. The
resulting molecular model shows mean unsigned deviations to experiment of 0.7%
in saturated liquid density, 1.6% in vapor pressure, and 2.7% in enthalpy of
vaporization over the whole temperature range from triple point to critical
point. This new molecular model is used to predict thermophysical properties in
the liquid, vapor and supercritical region, which are in excellent agreement
with a high precision equation of state, that was optimized to 1147
experimental data sets. Furthermore, it is also capable to predict the radial
distribution functions properly, while no structural information is used in the
optimization procedure.
