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 | a ℓ ℓ ′ | . For large values of | a ℓ ℓ ′ | corrections must be introduced mostly for the dipole–dipole term, the correct values of C6 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 = 10a0 and T = 293 K the electrostatic dipole–dipole component is dominant for ∣a11∣ > 0.5. For centrosymmetric molecules the corresponding electrostatic contribution is usually negligible with respect to dispersion.