Ab initiocalculation of the dipole moment of He-Ar and the collision-induced absorption spectra

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Abstract
For the study of induced dipole moments, the collisional He-Ar complex is treated as a molecule in self-consistent-field (SCF) and size-consistent coupled-electron-pair approximation (CEPA) calculations. The basis superposition error is effectively avoided by using nonorthogonal local orbital sets. The results suggest that the SCF plus approximate dispersion dipoles commonly considered may be at best in accidental agreement with experiment because of fortuitous cancellation: for He-Ar, the hitherto neglected intra-atomic correlation increases the exchange dipole by as much as 30% while the dispersion dipole (which has the opposite polarity of the exchange dipole) is almost 3 times larger than a perturbation estimate of the leading term has indicated. Collision-induced absorption spectra are computed on the basis of the ab initio dipole-moment function, and compared with Bosomworth and Gush’s celebrated measurement. We observe close agreement at frequencies where the absorption is strong. Only at the high and low ends of the measured frequency range does the measurement fall below theory by up to 20%, possibly because of the greater experimental uncertainties associated with weak absorption. Other measurements at relatively high densities have resulted in integrated two-body absorption intensities in reasonable agreement with our binary theory.