arXiv2003
We make use of a recently developed method to, not only obtain the exactly known eigenstates and eigenvalues of a number of quasi-exactly solvable Hamiltonians, but also construct a convergent approximation scheme for locating those levels, not amenable to analytical treatments. The fact that, the above method yields an expansion of the wave functions in terms of corresponding energies, enables one to treat energy as a variational parameter, which can be effectively used for the identification of the eigenstates. It is particularly useful for the quasi-exactly solvable systems, where the ground state is known and a number of eigenstates are bounded, both below and above. The efficacy of the procedure is illustrated by obtaining, the low-lying excited states of a prototypical double-well potential, where the conventional techniques are not very reliable. Our approach yields the approximate eigenfunctions and eigenvalues, whose accuracy can be improved to any desired level, in a controlled manner. Comparing the present results with those of an independent numerical method, it was found that, the first few terms in our approximate solutions are enough to yield the excited state eigenvalues, accurate upto the third place of the decimal.