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Experiments, quantum mechanics and molecular dynamics of high pressure hydrogen

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In this talk, I will discuss recent work on high pressure hydrogen at an accessible level, in particular relating density functional calculations to spectroscopic measurements.

In 1935, Wigner and Huntington used the recently-derived quantum theory of nearly free electrons to predict that hydrogen would become metallic under high pressure. Their calculation of lattice energy is in good agreement with modern methods. A massive underestimate of the required pressure has inspired generations of experimentalists to claim synthesis of solid metallic hydrogen: the aptly named “Holy Grail” of high pressure physics. Further theoretical predictions of zero-temperature melting and room temperature superconductivity has excited the search. At high temperatures, a negative-sloped melting curve and a transition to a metallic liquid provides further intrigue.

To date, five solid phases of hydrogen are reported, and in none of them are the atomic positions known. In part this is because the crystal structures are complex, but also because for highly quantum nuclei even the concept of “atomic position” is moot. Despite the obviously quantum nature of the system, and the large zero-point energies, treating the atoms classically in density functional calculation yields a phase diagram in remarkably good agreement with the experiment. It fails quantitatively in two cases: the phase I-II transition where classical rotors are arrested faster than quantum ones, and in the liquid-liquid transition where quantum vibrations break covalent bonds more effectively than classical one. Elsewhere, the correspondence principle holds good.

The seminal work of Pickard and Needs mapped out the low temperature structures, showing a series of phase transformations due (in my interpretation) to quadrupole interactions, efficient molecular packing, molecular metallisation and atomisation. At higher temperatures, the atoms are far more mobile and their average position does not correspond to minima of the energy surface. We used molecular dynamics to study the phases I, IV and V and the melt, finding good agreement with the experiments and providing insights into the structure and dynamics of the atoms.

This talk is part of the Materials Modelling Seminars series.

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