Eigenchannel treatment of R-matrix theory

We here present an alternatve way to perform R-matrix calculations in which the eigenchannel parameters of multichannel quantum defect theory are calculated directly. Merits of our R-matrix calculations are twofold: (1) an analytical treatment of perturbed discrete Rydberg states, autoionizing states, and their adjacent continuum to avoid numerical calculations outside the R-matrix box; (2) clear assignment of overlapped resonances, e.g., to analyze the lithium inner-shell photoionization cross section, which has been of considerable interest in recent experimental measurements.