Resonance information from the analytically continued S-matrix

The J-matrix method of scattering is used to calculate the scattering S-matrix at the set of energy eigenvalues of the full Hamiltonian matrix constructed from a finite set of square-integrable basis functions. The S-matrix is then analytically continued in the complex energy plane via a point-wise rational fraction scheme of Schlessinger. Numerical search techniques are then used to locate the poles of the S-matrix, which are identified with the resonance energies. Partial widths are easily calculated from the residues of the S-matrix at the designated complex resonance energies.