Resonances in the R-Matrix Method

The R-matrix method is widely used in scattering calculations. We present a simple extension that provides the energy and width of resonances by computing eigenvalues of a complex symmetric matrix. We briefly present the method and show some typical applications in two- and three-body systems. In particular, we discuss in more detail the \(^6\) ${}^6$ He and \(^6\) ${}^6$ Be three-body nuclei ( \(\alpha +n+n\) $\alpha +n+n$ and \(\alpha +p+p\) $\alpha +p+p$ , respectively). We show that large bases are necessary to reach convergence of the bound-state or resonance properties.