Molecular theory of hyperfine pressure shifts in H caused by Ar and He buffers

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Abstract
The general applicability of molecular methods for studying hyperfine pressure shifts (HPS) in a hydrogen atom caused by Ar and He noble-gas buffers has been tested thoroughly by carrying out extensive multiconfigurational self-consistent-field (MCSCF) and single-configuration self-consistent-field (SCF) calculations on ArH and HeH diatomic systems. Calculations show that for a He atom, a light noble-gas buffer, single-configuration SCF molecular wave functions are sufficiently good in reproducing the HPS result quantitatively, although qualitatively the weak, long-range polarization features are not described. However, for a heavier noble-gas buffer, such as the Ar atom, the long-range polarization effects are the strong and dominant features in the ArH binary interaction. Because of a lack of a sufficient number of proper functions for describing the long-range polarization features, the single-configuration SCF molecular wave functions do not reproduce the long-range polarization effects adequately. For ArH, therefore, MCSCF molecular wave functions, constructed with a choice of configurations which describe the dominant polarizations, are employed to reproduce the HPS result quantitatively and the long-range features qualitatively. HPS results computed by using MCSCF molecular wave functions are in excellent agreement with the experimental result.