For cases with a greater than zero, the potential is an anharmonic single well potential. In this case, both the 0 → 1 and 1 → 2 energy gaps increase as a increases. This relatively easy trend can be captured by both our CMESs-based method and the conventional PES-based method. However, our method outperforms the conventional method with a significantly better agreement with the quantum references. In the large a limit, the quartic terms can be ignored and the potential essentially becomes an harmonic oscillator. According to the discussions on harmonic oscillator, both 0 → 1 and 1 → 2 gaps converge to the same value and all methods give the same and correct asymptotic result.