Spin-free quantum chemistry. XV. Spin-only neutron diffraction

The cross section for elastic neutron diffraction is analyzed for the spin-only case, in which the orbital contributions to the magnetic moment density are negligible. For systems specified by spin-free Hamiltonians, we show that the magnetic moment density is calculated from the unpaired electron density, a spin-free quantity, which is equivalent to the spin density. The computation of the unpaired electron density is outlined and examples are discussed. The scattering cross section for an infinite interaction range Heisenberg model exhibits a temperature dependence which parallels that of the spontaneous magnetization. With a knowledge of the unpaired electron density one may determine the magnetic space group symmetry.