Ionic contributions to lattice instabilities and phonon dispersion inLa2CuO4

We have applied the potential-induced breathing model, a nonempirical ionic model, to ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. The ionic model predicts the observed tetragonal-to-orthorhombic distortion, predicts a lower-symmetry ground state, and predicts a stable oxygen breathing mode. We find unstable phonon branches that we relate to phase transitions. Harmonically unstable double-well modes will couple to the charge density in the copper-oxide planes and possibly contribute to high-$T_c$ superconductivity. This ionic coupling is not included in recent calculations.