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

Visualising potential energy surfaces using dimensionality reduction

25 November 2021

Computational structure prediction has emerged as a highly successful approach to the discovery of new materials. Candidate structures are created by constructing the most stable configurations that can be adopted by a given...

The elements of life under pressure

1 July 2021

First-principles structure prediction sheds light on high-pressure compounds formed from carbon, hydrogen, nitrogen and oxygen.

AIRSS for battery cathode materials

15 June 2021

A team of researchers at Cambridge and University College London have developed a computational framework for battery cathode exploration based on ab initio random structure searching.

Anatase-like Grain Boundary Structure in Rutile Titanium Dioxide

30 April 2021

A collaboration between researchers at Cambridge and AIMR has shed light on grain boundary structures in titania.

Physics World Breakthrough of the Year finalists for 2020

17 December 2020

A paper coauthored by Chris Pickard and Bartomeu Monserrat has been selected as a Top 10 finalist in Physics World's breakthroughs of the year for 2020.

An upper limit for the speed of sound

17 November 2020

A collaboration involving Bartomeu Monserrat and Chris Pickard was featured on both the University website and the Department website .

New fellowships for Chuck

17 November 2020

Congratulations to Chuck Witt, who began recently as a Schmidt Science Fellow and as a Junior Research Fellow at Christ's College, Cambridge! The Schmidt award is intended to catalyze new research directions and...