The core electrons were replaced by the Vanderbilt ultrasoft pseudopotentials. A fictitious mass of 700 au was used for Kohn-Sham orbitals and a timestep of 0.14 fs (6 au) was used for integrating the equations of motion of nuclear and orbital degree of freedom in Car-Parrinello MD. The mass of all the hydrogen atoms was replaced by that of deuterium to ensure adiabatic separation between nuclei and orbital subsystems. MD simulations were carried out in canonical ensemble (NVT), whereas Nosé-Hoover chain (NHC) thermostats were applied on both the nuclei and orbitals with NHC mass parameters set to 2500 cm -1