A quadrupole oscillator as an integrable model

A quadrupole oscillator is presented as an integrable model in the Born-Oppenheimer formalism with an electronic Hamiltonian being the quadrupole tensor. The electronic states of present concern are associated with a doubly degenerate positive eigenvalue of the electronic Hamiltonian, and accordingly the nuclear Hamiltonian takes a $2\times 2$ matrix form. While the potential function for nuclear motion is proportional to $r^2$, the kinetic energy operator is rather complicated, containing coupling terms with a Berry connection through adiabatic approximation. The energy eigenvalues, which receive a modification by a Chern number, get closer to those for the 3D isotropic harmonic oscillator if the angular momentum quantum number becomes sufficiently large.