Next we try a quadratic polynomial of degree 2 for fM(). The resulting fitted curves are shown in the Supporting Information. Compared with Figure , the agreement with the reference potentials is improved. The quantitative performance, shown in Table , is markedly better than for the linear model function. Errors for the extrapolated interval (𝐸 ) are different from expected by extrapolating the errors in the lower intervals. Surprisingly, the highest-level guiding potential now shows the worst performance. Of course, eq. ( ) yields only a harmonic potential with this model function.