Intensities and rates in the spectral domain without eigenvectors

An exact quantum-mechanical formula is presented which relates energy level fluctuations of a complex multidimensional quantum system to its spectrum. The formalism presented here can be used to efficiently calculate the spectrum and linewidth (rate), without recourse to eigenvectors of the Hamiltonian matrix, and yields insights into the relation between level fluctuations and spectral features. (C) 1996 American Institute of Physics.