A cubic polynomial of order 3 is the simplest polynomial that can provide anharmonic results via eq. ( ). The resulting fitted curves are shown in the Supporting Information. As expected, the errors are smaller with the cubic model function than with the quadratic. The choice of guiding potential has a correspondingly smaller effect. The guided results are markedly better than unguided. For a quartic polynomial (order 4) the fitted curves are plotted in the Supporting Information and the quantitative errors are listed in Table . For the guided fitting, results are noticeably improved only for the extrapolated interval. The results are markedly improved for the direct fitting of eq. ( ). Result for higher-order polynomials are plotted in the Supporting Information and summarized in Table . Errors generally decline for all fitting methods, as expected.