Compressed gases and liquids containing molecules of T-d and D-infinity h symmetry absorb far-infrared radiation, due to transient dipole moments induced during molecular collisions. In earlier theoretical work on far-infrared absorption by CH4/N-2 mixtures, good agreement was obtained between calculated and experimental spectra at low frequencies, but at higher frequencies-from 250 to 650 cm(-1)-calculated absorption intensities fell significantly below the experimental values. In this work, we focus on an accurate determination of the long-range, collision-induced dipoles of T-d...D-infinity h pairs, including two polarization mechanisms not treated in the earlier line shape analysis : dispersion and nonuniformity in the local field gradient acting on the Td molecule. Since these mechanisms produce transitions with Delta J= +/-3 or +/-4 for CH4 and Delta J=0 or +/-2 for N-2, their inclusion is expected to increase the calculated absorption intensities in the high frequency wings for CH4/N-2 mixtures. This should improve agreement with the experimental spectra, and permit more accurate determination of anisotropic overlap terms in the collision-induced dipole. We give numerical values for the long-range dipole coefficients of CH4 or CF4 interacting with H-2, N-2, CO2, or CS2; the dipole coefficients have been derived with spherical-tenser methods and evaluated using single-molecule moments and susceptibilities from recent ab initio calculations or experiments. The dispersion dipoles are given rigorously in terms of integrals involving the imaginary-frequency polarizability alpha(i omega) and the hyperpolarizabilities beta(0;i omega,-i omega) and B(0;i omega,-i omega). To obtain numerical estimates for the dispersion dipoles, we have developed constant-ratio approximations that require only the static susceptibilities and Cg van der Waals coefficients.