Abstract:In the case of quasi-degeneracy, i.e., when there are states close in energy to the state considered, standard perturbation theory may lead to convergence problem. A typical example is the relativistic treatment of the fine structure of light heliumlike ions. In Many-BodyPerturbation Theory (MBPT) this problem can be treated by including the quasi-degenerate states in an extended model space. The mixing is then treated to all orders already in the zeroth-order wave function by diagonalizing an effective hamilt… Show more
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