Accounting of the van der Waals interactions in pratical molecular calculations with density functional theory was analyzed by an empirical method. The coefficients of C6 for pair interactions between carbon, nitrogen, and oxygen atoms were developed by the least-square fitting to the molecular coefficients obtained from the dipole oscillator strength distribution method. The empirical method with the damping function was showed to drop to zero smoothly, and provided a significant correction to both of the Becke's hybrid functional, the PW91 exchange and correlation functional.