Monte Carlo simulations for diatomic molecules were performed by using
a site-site pair potential model together with a triplet potential. T
o reduce the computer time, an effective triplet potential is proposed
. This potential expresses the effects of Axilrod-Teller three-body in
teractions on a pair of atoms, and is given as a simple function of de
nsity and the distance between the pair of atoms. The computer time fo
r Monte Carlo simulations is almost equivalent to that of fluids with
only the site-site pair potential. From comparisons of the Monte Carlo
simulations and the experimental data of oxygen, the potential parame
ters are determined.