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