STUDIES IN PERTURBATION THEORY VI. CONTRACTION OF SECULAR EQUATIONS VII. LOCALIZED PERTURBATION,

The partitioning technique for solving secular equation is discussed. It is shown that the orginal secular equation may be transformed to a contracted secular equation referring to the specific subspace under consideration. The technique may be applied to heteroatoms in a molecule, to central atoms in a crystal field, to chemical bonds in a molecular environment, and to discuss the questin of dressed and undressed particles. If the system under consideration is not complete, the addition of a complementary subspace leads to a new term in the Hamiltonian corresponding to a dressing of the system involved. It is shown that eigenvalue problems associated with contracted Hamiltonians may be solved by iteration procedures. Author