A method to calculate long range coefficients and dynamic polarizabilities by time dependent density functional theory (TDDFT) for van der waals coefficients C 6 was discussed. Polarizability and absorption cross section of rare gas atoms was made. The local spin-density approximation (LSDA) and the general gradient approximation (GGA) produced an incorrect range behavior in the potential and orbital energies obtained from LSDA and GGA. The van der Waals coefficient C 6was a parameter in determination of collision properties and trap loss in gases at very low temperature. The value of C 6 calculated by empirical correction factor came out within a probable error of 3%.