On the Origin of Spin-Hamiltonian Parameters

The various coupling constants which appear in the phenomenological spin Hamiltonian for an atom or molecule can, in principle, be determined from a knowledge of the electronic wavefunction. It is difficult to obtain a physical picture of their origin and interrelationship because of the complexity of the many-electron wavefunction. The problem may be greatly simplified by introducing certain standard density functions, of which the ``spin density'' is the most familiar. Only three types of function are required, all with a physical meaning which is readily visualized. Their significance in determining spin-Hamiltonian parameters is discussed and illustrated with reference to (i) hyperfine coupling (contact and dipole—dipole), (ii) nuclear spin—spin coupling, (iii) electron spin—spin coupling, (iv) g values.