An extended equation of state for gaseous methane determined from the speed of sound

We extended earlier calculations of the fourth virial coefficient to graphs that contain at most two triplet potentials thus providing values of this coefficient for molecules with weak to moderate three-body forces. When the fourth virial coefficient is combined in an equation of state method for methane, it is shown that the resulting procedure performs as well as one that was based on a lower-order approximation and applied to argon. Values of the fourth acoustic virial coefficients of methane obtained from the method are given.