Symmetry-Adapted Perturbation Theory: A General Approach

Perturbation theory plays a fundamental role in the determination of quantum mechanical states. Here we shall be concerned with stationary states, so our attention will be restricted only to the time-independent variant of such theory. Besides conventional treatments, described in any textbook on quantum mechanics, a renewed interest in general perturbation theory is to be recognized in more recent years [18,19], In a remarkable series of papers, entitled “Studies in Perturbation Theory” [10,20–32], Löwdin has emphasized the partitioning technique as a tool for solving secular equations associated with eigenvalue problems, putting in evidence its connection with various perturbation expansions.