When experimental methods yield only partial results it is the role of theory to complete the story. The central aim of our research group is to exploit the synergistic relationship that now exists between computation and experiment.

One of our research interests lies in devising new computational procedures to complete crystal structures obtained from limited experimental data. For instance, diffraction data collected under extreme conditions of temperature or pressure can be of very poor quality, with the latter yielding only 30-40% of the data collected by routine analysis. The resulting structures tend to report only the locations of heavy atoms, and even then with large uncertainties.

A recent example is the high pressure polymorph of nitric acid dihydrate, which has been postulated as a possible catalyst for the breakdown of ozone in polar stratospheric clouds. This structure presented a particular challenge to complete as one of the two water molecules is actually present as an oxonium ion [H₃O]+. The experimental data only reported the location of the two oxygen atoms (given as the red dots in the left hand figure). Our modelling study revealed the existence of only one stable structure (right hand figure), and following the subsequent collection of high pressure neutron diffraction data, was proved to be correct.

We are also interested in molecular materials that exhibit proton migration – a subtle effect where a hydrogen atom located in a short, strong hydrogen bond changes its position with temperature and pressure. We demonstrated that ab initio molecular dynamics calculations could mimic this behaviour, which then allows for full interrogation of the computational model to discover why the phenomenon occurs.

Finally, a new direction for the group is the study of proton transport through models of transmembrane proteins.

SELECTED RECENT PUBLICATIONS

1. Nitric acid dihydrate at high pressure: An experimental and computational study M. Walker, C.R. Pulham, C. A. Morrison, D. R. Allan and W. G. Marshall, Phys. Rev., 2006, B73, 224110.
2. Towards understanding mobile proton behaviour from first principles calculation: The short hydrogen bond in urea-phosphoric acid C. A. Morrison, M. M. Siddick, P. J. Camp and C. C. Wilson, J. Am. Chem. Soc., 2005, 127, 4042.
3. Density functional calculations: Dihydrogen bonds in solid BH3NH3 C. A. Morrison and M. M. Siddick, Angew. Chem. Int. Ed., 2004, 43, 4780.