Hydrogen and helium are the most abundant elements in the Universe. They are also, in principle, the most simple. Nonetheless, they display remarkable properties under extreme conditions of pressure and temperature that have fascinated theoreticians and experimentalists for over a century. Advances in computational methods have made it possible to elucidate ever more of their properties. Some of these methods that have been applied in recent years, in particular, those that perform simulations directly from the physical picture of electrons and ions, such as density functional theory and quantum Monte Carlo are reviewed. The predictions from such methods as applied to the phase diagram of hydrogen, with particular focus on the solid phases and the liquid-liquid transition are discussed. The predictions of ordered quantum states, including the possibilities of a low- or zero-temperature quantum fluid and high-temperature superconductivity are also considered. Finally, pure helium and hydrogen-helium mixtures, the latter which has particular relevance to planetary physics, are discussed.

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The properties of hydrogen and helium under extreme conditions

Recent high-pressure studies reveal a wealth of new information about the behavior of molecular materials subjected to pressures well into the multimegabar range (several hundred gigapascal), corresponding to compressions in excess of an order of magnitude. Under such conditions, bonding patterns established for molecular systems near ambient conditions change dramatically, causing profound effects on numerous physical and chemical properties and leading to the formation of new classes of materials. Representative systems are examined to illustrate key phenomena, including the evolution of structure and bonding with compression; pressure-induced phase transitions and chemical reactions; pressure-tuning of vibrational dynamics, quantum effects, and excited electronic states; and novel states of electronic and magnetic order. Examples are taken from simple elemental molecules (e.g. homonuclear diatomics), simple heteronuclear species, hydrogen-bonded systems (including H2O), simple molecular mixtures, and s...

Effects of high pressure on molecules

Single-particle rotations in molecular crystals

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The Properties of Hydrogen and Helium Under Extreme Conditions

International Conference on the Physics of Semiconductors

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Ground State Energy Difference of Hcp and Fcc Ortho-Hydrogens

Two-dimensional angular distribution patterns of photoemission are calculated on the band picture for $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ for the normal incidence excitation by linearly and circularly polarized lights. The calculated results for the linearly polarized lights with $x$ and $y$ polarization perpendicular to the crystal $c$ axis ($z$ axis) can explain the experimental results of the linear dichroism obtained by the two-dimensional hemispherical mirror analyzer. The behavior of the calculated circular dichroism is discussed.

Linear and circular dichroism in photoemission angular distribution from the valence band of 1 T − TaS 2

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Hiroshi Miyagi

Papers

Ground State Energy Difference of Hcp and Fcc Ortho-Hydrogens

Linear and circular dichroism in photoemission angular distribution from the valence band of 1 T − TaS 2

Spin Correlation Effect on the Slow Neutron Scattering by Polyatomic Molecules with Particular Reference to Methane

Structural properties of molecular solid iodine under pressure: First-principles study of Raman-active A g modes and hyperfine parameters

Energy Levels of Triangular Clusters of Ortho-Hydrogen Molecules due to Quadrupole Interaction in Crystals