The equation of state of a real gas is given by \(\left(\mathrm{P}+\frac{\mathrm{a}}{\mathrm{V}^2}\right)(\mathrm{V}-\mathrm{b})=\mathrm{RT}\), where P, V and T are pressure. volume and temperature respectively and R is the universal gas constant.  The dimensions of \( \frac {a}{{b}^2} \) is similar to that of: