A semi-empirical equation of state is presented for the liquid rubidium metal. The Lennard-Jones (8.5-4) potential model, which originally has been derived for liquid cesium metal, is found to be suitable for modeling of liquid rubidium metal as well. By applying the experimental PVT data of compressed liquid metal in the range 500-1600 K, the slope B, and intercept C, of the linear isotherms are determined and accordingly parameters of the potential function are calculated. The contribution of non-spherical part of the interaction potential to the second virial coefficient B-2(ns), is calculated by using the Boltzman factor that involves the proposed model potential. The multipole moments, standing as approximation of the non-spherical contribution by multipole expansion, are calculated by the Gaussian 98W program at the B3LYP level of theory. It can be concluded that the slope of the isotherm conforms to B-2(ns) quite well, though some deviations at low T's exist. (c) 2006 Elsevier B.V. All rights reserved.