Molecular and Metallic Hydrogen

A comprehensive review of published data on metallic hydrogen summarizing properties of molecular hydrogen required to determine the molecular-to-metallic hydrogen transition pressure. The best available effective interaction potential is used in calculating the equation of state for solid molecular hydrogen. The equation of state of metallic hydrogen is determined by four different methods and the possible range of molecular-to-metallic hydrogen is largely responsible for the wide discrepancy in calculations of the transition pressure. Metastability of metallic hydrogen is discussed and experimental high-pressure research pertinent to determination of molecular-to-metallic hydrogen transition pressure is reviewed. The recently proposed concept of molecular-insulating phase becoming a molecular-conducting phase due to narrowing and closing the band gap under high pressure provides a more likely explanation for experimentally observed decrease in electrical resistivity of molecular hydrogen in both static and shock-wave experiments and attributed to molecular-to-metallic transition of molecular hydrogen.