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Background: The information about the 6Li(d,a)4He reaction rates of theastrophysical interest can be obtained by extrapolating direct data to thelower energies, or by indirect methods. The indirect Trojan Horse method, aswell as various R-matrix and polynomial fits to direct data, estimate theelectron screening energies much larger than the adiabatic limit. Calculationsthat include the subthreshold resonance estimate smaller screening energies.  Purpose: Obtain the 6Li(d,a)4He reaction R-matrix parameters and theastrophysical S factor for the energies relevant to the stellar plasmas byfitting the R-matrix formulas for the subthreshold resonances to the S factordata above 60 keV.  Methods: The bare S factor is calculated using the single and the two-levelR-matrix formulas for the closest to the threshold 0+ and 2+ subthresholdstates at 22.2, 20.2 and 20.1 MeV. The electron screening potential Ue is thenobtained by fitting it as a single parameter to the low energy data.  Results: The low energy S factor is dominated by the 2+ subthresholdresonance at 22.2 MeV. The influence of the other two subthreshold states issmall. R-matrix fits result in the electron screening that is smaller than theadiabatic value. Neglecting the electron screening above 60 keV reduces theelectron screening potential significantly. Calculations show a large ambiguityassociated with a choice of the initial channel radius.  Conclusions: The R matrix fits do not show a significantly larger Ue thanpredicted by the atomic physics models. The R-matrix best fit produces Ue=149.5eV and Sb(0)=21.7 MeV b.

Low-energy R-matrix fits for the 6Li(d,a)4He S factor