Abstract
A critical discussion of the theory of spin interactions in molecules is presented. We emphasize that the conventional spin hamiltonian can only be used in first-order perturbation theory, otherwise a singular hamiltonian theory is obtained. The (singular) perturbation of atomic states by the contact operator and an inverse square potential is discussed in detail. The introduction of a spatially extended charge and magnetization for nuclei is suggested as a way of developing a mathematically satisfactory theory of electron-nuclear spin coupling effects; an analogous phenomenology for electrons alone however is not possible.

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