Car-Parrinello quantum mechanical molecular dynamics were performed using plane wave basis sets in Quantum Espresso, version 6.6. After electronic minimization, both the ionic coordinates and cell size were relaxed using damped dynamics, with a force convergence threshold of 1.0 × e -4 a.u. Next, the vanishing field polarization was calculated before applying a homogeneous electric field of 0.001 a.u. Finally, the ionic and electronic degrees of freedom were relaxed using damping, until the polarization had converged.