This noncubic EOS consists of a repulsive and an attractive term. The repulsive term is Boublík's hard-convex-body EOS, which means that the BACK EOS can deal with nonspherical molecules. In order to improve the representation of real (soft) molecules, the hard-body volume is made temperature-dependent.
The attractive term is a 2-dimensional polynomial fitted to computer simulation data for the square-well fluid. The underlying computer simulations covered a wide range of states, but the shape of the polynomial does not ensure that extrapolation to high temperatures beyond the fitting range will give physically meaningful results. Furthermore, the polynomial shows oscillations; as a consequence, pressure isotherms can run to negative pressures at high densities.
adjustable parameters: 5
 S. S. Chen and A. Kreglewski, Ber. Bunsenges. Phys. Chem. 81 (1977) 1048–1052.
 J. J. Simnick, H. M. Lin, and K. C. Chao, in: ACS Adv. Chem. Ser., vol. 182, American Chemical Society, Washington 1979, 209–233.
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