Evaluation of hyperfine coupling tensors of the BeH and BeF radicals

Using the multiconfigurational self-consistent field restricted-unrestricted response function approach and following a systematic basis set and electron correlation study, accurate hyperfine coupling tensors have been determined for the X 2Σ states of the BeH and the BeF radicals. The basis set investigation started with Widmark et al. bases, for which systematic uncontractions and addition of diffuse and tight functions were carried out until saturation. For the BeH radical the selected basis gives hct that are in agreement with numerical Hartree-Fock results. The electron correlation study consisted of systematic extensions of the CAS valence spaces based on natural orbital occupation numbers. The final results agree well with available experimental data and clearly show that the BeF molecules are freely rotating in the argon and krypton matrices.