Local approximation to the critical parameters of quantum wells

We calculate the critical parameters for some simple quantum wells by means of the Riccati-Pad'{e} method. The original approach converges reasonably well for nonzero angular-momentum quantum number $l$ but rather too slowly for the s states. We therefore propose a simple modification that yields remarkably accurate results for the latter case. The rate of convergence of both methods increases with $l$ and decreases with the radial quantum number $n$. We compare RPM results with WKB ones for sufficiently large values of $l$. As illustrative examples we choose the one-dimensional and central-field Gaussian wells as well as the Yukawa potential. The application of perturbation theory by means of the RPM to a class of rational potentials yields interesting and baffling unphysical results.