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 $\rho_{\mathrm{max}}=\infty$.