We use the standard
expression for the second derivative of a function \( u \)
$$
\begin{equation}
u''=\frac{u(\rho+h) -2u(\rho) +u(\rho-h)}{h^2} +O(h^2),
\tag{6}
\end{equation}
$$
where \( h \) is our step.
Next we define minimum and maximum values for the variable \( \rho \),
\( \rho_{\mathrm{min}}=0 \) and \( \rho_{\mathrm{max}} \), respectively.
You need to check your results for the energies against different values
\( \rho_{\mathrm{max}} \), since we cannot set
\( \rho_{\mathrm{max}}=\infty \).