The Lennard-Jones potential and Barker-Henderson perturbationtheory are used to calculate the dispersion and repulsion energies between molecules. The dipole-dipole energy for polar molecules is treated as an average of space orientation. The Helmholtz free energy contribution of dipole moment is deduced from the perturbation theory also and tested by Monte Carlo molecular simulation data. The dipole-induced dipole energy is considered too. The component activity coefficients for H2O-nC4H9OH binary system are calculated. The results are satisfactory.