This paper illustrates the application of group theory to a
quantum-mechanical three-dimensional quartic anharmonic oscillator with Oh
symmetry. It is shown that group theory predicts the degeneracy of the energy
levels and facilitates the application of perturbation theory and the
Rayleigh-Ritz variational method as well as the interpretation of the results
in terms of the symmetry of the solutions . We show how to obtain suitable
symmetry-adapted basis sets