Martin Dove

In my time working as a post-doc with Stuart Pawley in Edinburgh, I was introduced to the triple-axis spectrometer for measurements of phonon dispersion curves. This was carried out with Brian Powell at Chalk River. The main system I looked as was the molecular crystal C₆F₃Cl₃, for which we determined the complete set of dispersion curves for the rigid-body motions along the three main symmetry directions. Chalk River became my main source of neutrons for several years, and I spent a very happy year there in 1992-3. I performed a number of other inelastic scattering studies (particularly on SF₆, calcite and sodium nitrate), together with a number of other powder diffraction studies.

When I joined the Department of Earth Sciences, ISIS was just coming on line. My first use of ISIS, which continued for some years, was to perform high-resolution powder diffraction on the HPRD instrument. In collaboration with Mark Hagen and Mark Harris (then a PhD student in our group) I started to use the PRISMA spectrometer for measurements of phonons. We performed a detailed study of phonons in calcite, discovering an unusual excitation that we still do not understand. This led to a detailed triple-axis spectrometer experiment, demonstrating (to me at least) the complementarity of the two types of instrument.

I also explored two other inelastic spectrometers at ISIS. Ian Swainson and I used the crystal analyser spectrometer TFXA (since replaced by TOSCA) to compared the low-energy excitations in the two phases of cristobalite, an experiment which gave us our first dramatic demonstration of rigid unit modes in the high-temperature phase. Björn Winker, Tina Line and I started a project using incoherent quasielastic scattering to study the motions of water in minerals. We did some work on the mineral analcime, NaAlSi^₂O^₆.H^₂O using the IRIS spectrometer at ISIS. This gave some nice results. Unfortunately this was at the time that the ILL was closed down for a major refit, and the loss of the cold-neutron spectrometers at ILL meant that IRIS was the only spectrometer of its type in Europe. Demand on the instrument was too great to enable us to develop a programme of work. It is to be hoped that someone will pick up this line of work some time, because I feel that this type of experiment offers a unique probe of the motions of water molecules in minerals.

During my year in Chalk River I started to think about total scattering experiments as a possible means to gain insights into the structures of disordered crystalline materials. I subsequently teamed up with Dave Keen and Alex Hannon at ISIS to perform some experiments on the disordered phases of cristobalite and other silica polymorphs, using the LAD diffractometer. Following the success of this work, we obtained funding from EPSRC to develop a programme of work in this area, now based on the new GEM diffractometer. GEM is ideal for this work, combining the ability to collect high-quality total scattering data at the same time as high-quality diffraction data. We have adapted the standard RMC codes for the study of crystalline materials on time-of-flight sources; to my mind RMC has the potential to give significant structural information about structure over a wide range of length scales.

With initial support from NERC, followed by funding from the Royal Society and the Leverhulme Trust, a group of us have been developing a programme of work to study the structure of materials at simultaneous high temperatures and pressures. We have succeeded in reaching 7 GPa and 1500 K, and are now working to stretch these limits. We are using neutron radiography to measure temperature.

We have also recently carried out some studies using the MARI chopper spectrometer at ISIS. These studies have been aimed at looking at the comparison between amorphous and crystalline silica phases.

Finally, I am a member of a consortium with Keith McEwen (UCL), Stephen Hayden (Bristol) and Roger Eccleston (ISIS) that was awarded three grants of around £1.3M each to build a new high-intensity chopper spectrometer at ISIS, to be called MERLIN.

I am pleased to acknowledge collaboration with Stuart Pawley, Brian Powell, Ian Swainson, Mark Hagen, Mark Harris, Tina Line, Björn Winkler, Matt Tucker, Dave Keen and Steve Bennington. This research has been supported by the NERC and the EPSRC.

My interest in computer simulation took off in Edinburgh working with Stuard Pawley. First I was introduced to his molecular lattice dynamics code, but then we worked on molecular dynamics methods for orientationally disordered crystals. It was an exciting time to be in Edinburgh, because Stuart had introduced us to the ICL Distributed Array Processor (DAP), a machine with 4096 parallel processors that would perform identical operations on 4096 sets of data. Eventually Edinburgh was able to purchase two DAPs of its own. We were able to perform simulations on samples that were an order of magnitude larger than others were using at the time.

When I moved to Cambridge I continued in this line of work, and continued to use the DAP technology in London. To continue this story, a consortium of DAP users in Cambridge received grant funding to allow us to purchase two DAPs of our own. This consortium then formed the original nucleus of a new consortium that built up the Cambridge High Performance Computing Facility (HPCF), which started with a Hitachi parallel machine and a vector machine, and then from a series of large grants we were able to purchase large parallel computers from Silicon Graphics and IBM. The latest stage is a 1.5 TFlop machine from SUN.

When I joined Earth Sciences, I started to work on modelling of silicates, using lattice energy, lattice dynamics and molecular dynamics. Initially our lattice energy and lattice dynamics calculations were performed on a DEC MicroVAX 2000, which was quite reasonable for its time, but tasks that took one hour now take seconds on our Silicon Graphics workstations. We developed our own molecular dynamics code for simulations of silicates on the DAP, and also developed our own Monte Carlo code for simulations of cation ordering for the DAP. We now use the standard DL_POLY code for our molecular dynamics work, and we have written our own Monte Carlo code called OSSIA which is specifically designed for running on parallel machines.

All this work is performed using empirical model potentials. These have a number of significant advantages, particularly in allowing us to use large samples, to perform many calculations, and to perform molecular dynamics simulations. Empirical models can be made to work really well, but they have limitations that we are able to understand. When we need to go beyond empirical models, we use DFT ab initio methods. Initially we worked with the CASTEP code. Recently we started work with the order-N DFT code called SIESTA. This work has received a significant boost by our recruitment of Emilio Artacho onto our faculty staff.

The other type of simulation method we are using is the Reverse Monte Carlo method for building configurations of atoms from neutron total scattering measurements (see left).

My main interest in using computer simulations has been to study phase transitions, but this has extended to other areas of science (see below). In some cases we can use very simple tools to give answers to scientific questions, but in other cases we need to use much more sophisticated tools. Examples of recent studies include

• cation ordering, using a combination of lattice energy calculations (with empirical and DFT models) and Monte Carlo methods
• temperature and pressure dependence of glasses using molecular dynamics
• simulations of high-energy radiation damage in crystals
• simulations of organic molecules adsorped on mineral surfaces
• diffusion of cations on domain walls

Most recently we have been funded by NERC to develop an escience/Grid approach to molecular simulations of environmental processes. This will see the development of methods and ways of working to enable us to run much larger simulations.

I am pleased to acknowledge collaboration with Volker Heine, Andrew Giddy, Kenton Hammonds, Alix Pryde, Manoj Gambhir, Matt Tucker, Dave Keen, Kostya Trachenko, Mark Calleja, William Lee, Stephen Wells, Erika Palin, Tom Archer and Sally Birse. This research has been supported by the NERC and the EPSRC.