Parametric Statistics of Individual Energy Levels in Random Hamiltonians
  We establish a general framework to explore parametric statistics of individual energy levels in disordered and chaotic quantum systems of unitary symmetry. The method is applied to the calculation of the universal intra-level parametric velocity correlation function and the distribution of level shifts under the influence of an arbitrary external perturbation. 
I. E. Smolyarenko and B. D. Simons