Lemus, R., A. Frank, R. Bijker, F. Pérez Bernal, ., and J. Arias. “A Local Anharmonic Treatment of Vibrations of Methane”. Revista Mexicana De Física, vol. 45, no. 2, Jan. 1999, pp. 125-31, https://rmf.smf.mx/ojs/index.php/rmf/article/view/2836.