skip to content

Materials Modelling Seminar

Professor Graeme Ackland, School of Physics and Astronomy, University of Edinburgh

Thursday 21st June, 12:00

Goldsmiths 1 (0_017), Department of Materials Science & Metallurgy

Title: Experiments, quantum mechanics and molecular dynamics of high pressure hydrogen


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.

Latest news

Fast and easy exploration of crystal properties using machine-learned Ephemeral Data-derived Potentials

12 January 2024

Machine learning is quickly gaining prominence in the field of computational materials science. In the Materials Theory Group, we develop so-called ‘machine learned interatomic potentials’ (MLIPs), which can describe the...

Structure and colour in nitrogen-doped lutetium hydride

19 December 2023

Superconducting materials have a wide range of applications - from efficient power transmission to the advanced electromagnetics used in MRI machines - due to their loss-free conductivity. Current practical superconductors...

Quantum-induced hydrogen hopping in high-temperature superconducting lanthanum polyhydride

14 April 2023

Figure caption : Quantum effects are essential for hydrogen to dynamically explore different configurations. On the left, we see how the hydrogen atoms cover much larger distances at all temperatures when quantum effects are...

Flat water and ice

26 September 2022

Figure Caption : Pentagonal ice – a two-dimensional form of ice predicted to form when water is squeezed between graphene sheets. Water can be found trapped in nanoscale cavities, for example in biological membranes, or in...

Congratulations Ben Shires!

2 August 2022

Ben completed his PhD viva last week, covering his work on SHEAP , and he will soon be Dr Shires. Congratulations! shires.jpg

Quicker Crystals

14 July 2022

First-principles structure prediction has enabled the computational discovery of materials with extreme, or exotic properties. For example, the dense hydrides, which following computational searches have been found to...

New carbonates uncovered

7 January 2022

A study by Joseph Nelson and Chris Pickard of the Department of Materials Science and Metallurgy, University of Cambridge and the AIMR, Tohoku University, uses structure prediction to exhaustively explore the Ti-C-O and Al-C-O ternary systems.