Keesom coefficients in gases

T-dependent long-range Keesom coefficients are evaluated up to the R-10 term for small values of the dimensionless parameter vertical bar a(ll ')vertical bar For large values of vertical bar all 'vertical bar, I corrections must be introduced mostly for the dipole-dipole term, the correct values of Q being best obtained from a recently derived asymptotic formula. The corresponding attractive energies are the isotropic electrostatic contributions to the interaction energy and are temperature-dependent. Comparison with long-range induction and dispersion energy results for some simple polar axially symmetric molecules in the gas phase shows that at R = 10a(0) and T = 293 K the electrostatic dipole-dipole component is dominant for vertical bar a(11)vertical bar > 0.5. For centrosymmetric molecules the corresponding electrostatic contribution is usually negligible with respect to dispersion.