Spin-free quantum chemistry. XVI. Spin correlation

The calculation of quasielastic critical neutron scattering for systems defined by a spin-free Hamiltonian is discussed. The system dependent property which determines the critical scattering is the thermal average of a spin-free operator called the general unpaired electron correlation operator. For localized models, the scattering is obtained as the Fourier transform of an unpaired site correlation function. The algebraic relationship between this correlation function and the usual spin correlation function is discussed as are its relationships to thermodynamic properties. Correlations in the infinite interaction range model, linear spin wave model, and infinite chain Heisenberg model are given. In the last case, an estimate of the low temperature correlation length is made. (U.S.)