and fig. \[F:2\]$(b)$. One can try to approximate the critical potential, as usual, with a square well, but now the flat bottom of the well should extend over a finite distance in $\beta_2$ and $\beta_3$, and be symmetric in $\beta_3 $ around $\beta_3=0$. The shape of the borders is obviously relevant to the result. One could imagine shapes like those shown in fig. \[F:3\] with dashed or dotted lines, but their description would involve at least two or three free parameters, and the comparison with experimental data could be not very significant. We have found, however, that good results are obtained also with a simple rectangular shape (solid line in fig. \[F:3\]), implying only one free parameter, $b=\beta_3^w/\beta_2^w$ (apart from a common factor of scale).

![(color on line) Possible shapes for a potential well simulating the critical–point potential. The potential–energy surface of fig. \[F:2\]$(b)$ is also shown for comparison.[]{data-label="F:3"}](biz_f3.ps){height="5.9cm"}

\[S:4\]Results and comparison with experimental data
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\[S:4.1\] The Energy eigenvalues
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Now, as a first step, we can evaluate, as a function of $b$, the level energies in the ground–state band and deduce the best value of the parameter from a comparison with experimental results (fig. \[F:5\]).

![image](biz_f4.ps){height="\linewidth"}

To proceed, 