The thermal expansion, Hugoniot state and 300 K isotherm of sodium have been calculated on the basis of: (i) the accurate calculations of OK total energies with the full-potential linearized augmented plane wave method within the generalized gradient approximation to exchange-correlational functional; and (ii) the newly developed classical mean-field statistics where both the cold and thermal parts of the Helmholtz free-energy are entirely derived from the OK total energy. A quite satisfactory agreement between calculation and experiment has been observed. Our approach does not invoke any empirical parameter, which has long been desirable for the field of material science.