Zero-Field Splitting of Molecular Zeeman Levels

Electron spin-spin coupling, responsible for zero-field splitting of Zeeman levels, may be discussed very generally by means of a ``coupling anisotropy function'' defined in terms of the components of the two-electron density matrix. The properties of this function are fully discussed elsewhere: the present note is concerned with zero-field splitting of triplet levels and with the series of approximations which lead from the general results to simple (orbital theory) approximations. A numerical calculation on the phosphorescent triplet states of naphthalene gives reasonable agreement with experiment.