Heat And Thermodynamics

\(\left(P+\frac{a}{V^{2}}\right)(V-b)=R T\) represents the equation of state of some gases. Where \(P\) is the pressure, \(V\) is the volume, \(T\) is the temperature and \(a, b, R\) are the constants. The physical quantity, which has dimensional formula as that of \(\frac{b^{2}}{a}\), will be: