We compare the effect of a spin-orbit term and a Peierls term on the ground state of the half-filled hopping Hamiltonian. These terms are caused by displacement on the background to the lattice sites (spin-orbit term) and displacement on the actual lattice sites (Peierls term). We show how Peierls-like instabilities can result in either case and explain why the spin-orbit model can be more singular than the Peierls lmodel. We disregard electron-electron interactions and apply our results in two dimensions to ${\mathrm{La}}_2$${\mathrm{CuO}}_4$.

• Received 25 August 1994

DOI:https://doi.org/10.1103/PhysRevB.51.6892