  The coupling parameter expansion in thermodynamic perturbation theory of
simple fluids is generalized to include the derivatives of bridge function. We
applied seventh order version of the theory to Square-Well (SW) and
Lennard-Jones (LJ) fluids using Sarkisov Bridge function. In both cases, the
theory reproduced the radial distribution functions obtained from integral
equation theory (IET) and simulations with good accuracy. Also, the method
worked inside the liquid-vapor coexistence region where the IETs are known to
fail. In the case of SW fluids, the use of Carnahan-Starling expression for
free energy density of Hard-Sphere reference system has improved the
liquid-vapor phase diagram (LVPD) over that obtained from IET. We also obtained
the surface tension of SW fluids of various ranges. Results of present theory
and simulations are in good agreement. In the case of LJ fluids, the equation
of state obtained from the present method matched with that obtained from IET
with negligible deviation. We also obtained LVPD of LJ fluid from virial and
energy routes and found that there is slight inconsistency between the two
routes. The applications lead to the following conclusions. In cases where
reference system properties are known accurately, the present method gives
results which are very much improved over those obtained from the IET with the
same bridge function. In cases where reference system data is not available,
the method serves as an alternative way of solving the Ornstein-Zernike
equation with a given closure relation with the advantage that solution can be
obtained throughout the phase diagram with a proper choice of the reference
system.
