  Lattice stability and metastability, as well as melting, are important
features of the physics and chemistry of dense hydrogen. Using ab initio
molecular dynamics (AIMD), the classical superheating limit and melting line of
metallic hydrogen are investigated up to 1.5 TPa. The computations show that
the classical superheating degree is about 100 K, and the classical melting
curve becomes flat at a level of 350 K when beyond 500 GPa. This information
allows us to estimate the well depth and the potential barriers that must be
overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE)
using path integral molecular dynamics (PIMD) predicts that both superheating
limit and melting temperature are lowered to below room temperature, but the
latter never reach absolute zero. Detailed analysis indicates that the melting
is thermally activated, rather than driven by pure zero-point motion (ZPM).
This argument was further supported by extensive PIMD simulations,
demonstrating the stability of Fddd structure against liquefaction at low
temperatures.
