Exchange Interactions II: Spin Hamiltonians

In Part I the physical mechanism of exchange interactions have been discussed. In this part we introduce the general concept of spin-hamiltonian. Isotropic exchange hamiltonian for many-electron polynuclear clusters (Heisenberg-Dirac-van Vleck-HDVV model [1-6]) will be derived. Spin-hamiltonian approach allows to separate the full exchange problem into two independent ones: 1) evaluation of the energy levels of the exchange system considering Jij as a set of semiempirical parameters, and 2) quantum mechanical calculation of exchange parameters with the aim of elucidation of the main physical mechanisms of the exchange coupling. In this part we shall focus on the problem of calculation of spin levels in the framework of spin-hamiltonian approach.