Both temperature and pressure are maintained using the Nosé-Hoover chain thermostat (coupling frequency 0.01 fs -1 ), and barostat (coupling frequency 0.0005 fs -1 ). Electrostatic interactions have been treated by the Ewald summation technique with a cut-off radius of 12 Å, an inverse Gaussian charge width of 0.23 Å-1 , and 7×7×7 vectors for the reciprocal space.