## Peng-Robinson-Gasem equation (2001)

There have been many suggested improvements to the temperature dependence
function of the parameter *a* in the Peng-Robinson equation (e.g.
those of Mathias, Mathias-Copeman, and Twu and co-workers). The version
included in this program is the temperature dependence of Gasem, Gao,
Pan & Robinson described in
*Fluid Phase Equilibria*, **181**, 113-125 (2001). The function is:
This equation is one of the best equations of state that require just
three input parameters (*T*_{c}, *P*_{c}, and
*ω*). It is more accurate for heavy hydrocarbons and
works better at temperatures above the critical point than the normal
Peng-Robinson equation. Note, though, that it isn't necessarily better
than the SRK or PR equation for light hydrocarbons, and may even perform
worse than them in some cases.
Also, it is still not expected to be particularly accurate
when predicting properties of highly polar molecules, particularly those
that are capable of hydrogen bonding.

More sophisticated equations of state may be required for good predictions
of the properties of polar molecules over a wide range of conditions
(e.g. those of Patel & Teja or Stryjek & Vera). These equations, however,
normally require more than three input parameters in their formulation.