On the calculation of S-matrix poles using the Siegert method

The complex poles of the S matrix are determined by solving the Siegert eigenvalue problem using a variational method. Virtual states as well as resonances have been calculated. A boundary condition at a finite distance R is introduced, as proposed by Siegert. This requires that the potential be truncated at r=R. The error due to the fact that the potential does not vanish for r>R is corrected by a perturbative method. An analysis of the cut-off problem in the complex momentum plane is given.