  The pseudogap is a key property of the cuprate superconductors, whose
understanding should illuminate the pairing mechanism. Recent experimental data
support a close connection between the pseudogap and an oxygen-driven C4
symmetry breaking within the CuO2 plane unit cell. Using ab initio Molecular
Dynamics, we demonstrate the existence of a strong nonlinear electron-oxygen
vibrator coupling in two cuprates. In a mean field approach applied to this
coupling within a model Hamiltonian, we derive a C4 splitting/pseudogap phase
diagram in agreement with experiment - providing an explanation for the
pseudogap phenomenon from first principles. The implications for
superconductivity and the Fermi surface arc effect are discussed.
