We use the by now standard expression for the second derivative of a function \( u \) $$ \begin{equation} u''=\frac{u(\rho+h) -2u(\rho) +u(\rho-h)}{h^2} +O(h^2), \tag{3} \end{equation} $$ 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 \( \rho_{\mathrm{max}}=\infty \).