  A model Hamiltonian representing the Cu spins in La$_2$CuO$_4$ in its
low-temperature body-centred orthorhombic phase, that includes both spin-orbit
generated Dzyaloshinskii-Moriya interactions and interplanar exchange, is
examined within the RPA utilizing a Tyablikov decoupling of various high-order
Green's functions. The magnetic susceptibility is evaluated as a function of
temperature and the parameters quantifying these interactions, and compared to
recently obtained experimental data of Lavrov, Ando and collaborators. An
effective Hamiltonian corresponding to a simple tetragonal structure is shown
to reproduce both the magnon spectra and the susceptibility of the more
complicated body-centred orthorhombic model.
