Lower and upper bounds of the energy spectrum for potentials with multiminima

An extended Rayleigh–Ritz method for computing two‐sided eigenvalue bounds of the one‐dimensional Schrödinger equation without using a complementary method is presented. The method is based on the B‐spline approximation over the truncated domain, which results in a generalized banded matrix eigenvalue problem. Numerical results for two multiminima potentials of different natures are presented.