Partitioning in multiconfiguration perturbation theory



A simple and effective formulation of multi-configuration perturbation theory is reviewed emphasizing the various possibilities for partitioning the Hamiltonian. We study different principles how traditional partitionings (like that of Møller and Plesset or Epstein and Nesbet) can be generalized to the multi-configurational case. Level shift parameters are introduced with the purpose of fine-tuning the partitioning. Variational conditions for optimizing level shift parameters in multi-configuration theory are investigated, both for N-particle levels and for quasiparticle (one-electron) energies.