  A systematic investigation on vapor-liquid equilibria (VLE) of dipolar and
quadrupolar fluids is carried out by molecular simulation to develop a new
Helmholtz energy contribution for equations of state (EOS). Twelve two-center
Lennard-Jones plus point dipole and point quadrupole model fluids (2CLJDQ) are
studied for different reduced dipolar moments mu^{*2}=6,12, reduced quadrupolar
moments Q^{*2}=2,4 and reduced elongations L^*=0, 0.505,1. Temperatures cover a
wide range from about 55% to about 95% of the critical temperature of each
fluid. The NpT + test particle method is used for the calculation of vapor
pressure, saturated densities and saturated enthalpies.Critical data and the
acentric factor are obtained from fits to the simulation data.On the basis of
this data, an EOS contribution for the dipole-quadrupole cross-interactions of
non-spherical molecules is developed. The expression is based on a third-order
perturbation theory and the model constants are adjusted to the present 2CLJDQ
simulation results. When applied to mixtures, the model is found to be in
excellent agreement to results from simulation and experiment. The new EOS
contribution is also compatible with segment-based EOS, like the various forms
of the Statistical Associating Fluid Theory (SAFT) EOS.
