Perturbation theory, elementary models

Perturbation theory has been widely used in reactor physics, chiefly for the calculation of eigenvalues such as the change of reactivity to due some change or perturbation of reactor properties or configuration. Since the exact analytical solution of even the elementary diffusion model is limited... 

Ligand-field models serve the purpose of parameterizing experiments ). Their beauty and applicability stem from their derivation from the elementary theory of atomic spectra they are first-order perturbation models based upon a basis set of I functions (assumption 1, p. 71), and hence the interelectronic repulsion within the I shell may be accounted for in terms of Condon and Shortley parameters or Racah parameters. We obtain the expanded radial function model (25, 13,8). 

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