Extracting S-matrix poles for resonances from numerical scattering data: Type-II Padé reconstruction Original Research Article

We present a FORTRAN 77 code for evaluation of resonance pole positions and residues of a numerical scattering matrix element in the complex energy (CE) as well as in the complex angular momentum (CAM) planes. Analytical continuation of the S-matrix element is performed by constructing a type-II Padé approximant from given physical values (Bessis et al. (1994) ; Vrinceanu et al. (2000) ; Sokolovski and Msezane (2004) ). The algorithm involves iterative ‘preconditioning’ of the numerical data by extracting its rapidly oscillating potential phase component. The code has the capability of adding non-analytical noise to the numerical data in order to select ‘true’ physical poles, investigate their stability and evaluate the accuracy of the reconstruction. It has an option of employing multiple-precision (MPFUN) package (Bailey (1993) ) developed by D.H. Bailey wherever double precision calculations fail due to a large number of input partial waves (energies) involved. The code has been successfully tested on several models, as well as the F + H2 → HF + H, F + HD → HF + D, Cl + HCl → ClH + Cl and H + D2 → HD + D reactions. Some detailed examples are given in the text.