Calculation of collective energies from periodic time-dependent Hartree-Fock solutions

A periodic time-dependent Hartree-Fock solution is used as the reference state for a diagrammatic expansion of the propagator. A discrete Fourier transform leads to a function of energy whose poles are the corresponding energy levels. Limiting the expansion to first-order diagrams leads to a new derivation of the Bohr-Sommerfeld--type quantization rule for collective states