Ab Initio Simulations of Catalytic and Environmental Interfaces

Our research aims at understanding important phenomena in surface- materials- and nano-science. Using concepts from quantum mechanics to statistical mechanics, we apply and develop methods and computer simulations to study, for instance, chemical reactions at surfaces and processes of environmental relevance. Much of our research is carried out with leading experimentalists across the world. It is of both fundamental and applied interest. Water is a major focus of our work.

More information on our interests can be found below or by checking out some of our recent science highlights. A poster summarising some of our recent work can be found here.

Water at metal surfaces

Water covers almost all solid surfaces under ambient conditions. From heterogenous ice nucleation on aerosol particles to waste water treatment, interfacial water is of crucial importance to an endless list of problems in the physical and chemical sciences. A prerequisite to understand these varied phenomena is the seemingly simple task of establishing what the water overlayer structure is. However, characterizing water overlayer structures is a challenging task and despite thousands of publications on the chemical physics of water at interfaces only a handful of determinations have been accomplished to date. Some references (more) 1) J. Carrasco, B. Santra, J. Klimeš and A. Michaelides To wet or not to wet? Dispersion forces tip the balance for water-ice on metals Phys. Rev. Lett. 106, 026101 (2011) 2) J. Carrasco, A. Michaelides, M. Forster, S. Haq, R. Raval and A. Hodgson A one-dimensional ice structure built from pentagons Nature Mater. 8, 427 (2009)

Ice

One reason the group is called the ICE group is we do a lot of work trying to understand the structure and properties of ice under various conditions. This includes ice at very high pressure, the ice surfaces, and ice nanoparticles. Some references (more) 1) M. Watkins, D. Pan, E. G. Wang, A. Michaelides, J. VandeVondele and B. Slater Large variation of vacancy formation energies in the surface of crystalline ice Nature Mater. 10, 794-798 (2011) 2) D. Pan, L. M. Liu, B. Slater, A. Michaelides and E. Wang Melting the Ice: On the Relation between Melting Temperature and Size for Nanoscale Ice Crystals ACS Nano. 5, 4562 (2011)

Ice nucleation

How ice forms is a poorly understood phenomenon: although it may seem trivial to make ice by putting a bottle of the water in a domestic freezer, the liquid form can exist to temperatures below 0oC (this is known as supercooling and you can see a demonstration here). When ice forms at temperatures close to the melting point, it is almost always due to the presence solid impurities that act as ice nucleating agents. Understanding how the properties of these solid particles affect the nucleation mechanism is not only of industrial relevance (particularly in the aviation industry), but it is also important in the atmospheric sciences, where mineral dust from desert regions facilitates ice growth in the upper troposphere. Part of our research aims to further our understanding of ice nucleation by implementing a range of computational techniques, from DFT studies of small water clusters and layers at different surfaces, through to large-timescale molecular dynamics simulations where we directly probe the nucleation mechanism. Some references (more) 1) S. J. Cox, S. M. Kathmann, J. A. Purton, M. J. Gillan, A. Michaelides Non-hexagonal ice at hexagonal surfaces: the role of lattice mismatch Phys. Chem. Chem. Phys. 14, 7944 (2012) 2) D. Pan, L. M. Liu, G. A. Tribello, B. Slater, A. Michaelides and E. Wang Surface energy and surface proton order of the ice Ih basal and prism surfaces J. Phys.: Condensed Matter 22, 074209 (2010)

Heterogeneous catalysis

Our group has many years experience in the theory and simulation of chemical processes at surfaces, particularly those of relevance to heterogeneous catalysis. Some of the issues we are currently interested in are the oxide formation and clean, defective Au covered ceria surfaces. Some references (more) 1) C. Zhang, A. Michaelides, and S. J. Jenkins Theory of gold on ceria Phys. Chem. Chem. Phys. 13, 22 (2011) 2) T. J. Lawton, J. Carrasco, A. E. Baber, A. Michaelides and E. C. H. Sykes Visualization of Hydrogen-bonding and Associated Chirality in Methanol Hexamers Phys. Rev. Lett. 107, 256101 (2011)

Solid-liquid interfaces

Under ambient conditions, most surfaces are covered with a thin film of water. As such solid-water interfaces are of relevance to a huge array of scientific and technologies areas. Exciting recent advances in computational algorithms and hardware mean treat it is now possible to examine in intimate details structures and dynamics at solid-liquid interfaces entirely from first principle. Over the last few years we have studied a variety of systems such as water on salt, the controversed water/TiO2 interfaces and water on ZnO. Some references (more) 1) L. M. Liu, A. Laio and A. Michaelides Initial stages of salt dissolution determined with ab intio molecular dynamics Phys. Chem. Chem. Phys. 13, 13162 (2011) 2) L. M. Liu, C. Zhang, G. Thornton, and A. Michaelides Structure and dynamics of liquid water on rutile TiO2(110) Phys. Rev. B. 82, 161415(R) (2010)

Benchmarking DFT

Most of the "first principles" simulations we do are with a theory known as density-functional theory (DFT). In principle it is exact but in practice it relies on an approximation for how electrons interact with each other. We are tackling the issue of the accuracy of DFT through extensive series of studies of small gas phase complexes, and water-solid interactions. These benchmark studies with techniques such as Møller Plesset perturbation theory, coupled cluster, or quantum Monte Carlo often come with extreme computational burdens. However, these benchmarks are essential to establish the accuracy of more traditional methods such as DFT, and help to ensure that the numbers we produce stand the test of time and experiment. Some references (more) 1) B. Santra, J. Klimeš, D. Alfè, B. Slater, A. Michaelides, R. Car and M. Scheffler Hydrogen bonds and van der Waals forces in ice at ambient and high pressures Phys. Rev. Lett. 107, 185701 (2011) 2) B. Santra, A. Michaelides and M. Scheffler Coupled cluster benchmarks of water monomers and dimers extracted from DFT liquid water: the importance of monomer deformations J. Chem. Phys. 131, 124509 (2009)

Dispersion forces (van der Waals)

London dispersion interactions are ubiquitous in nature contributing to the binding of biomolecules such as DNA, molecular crystals, and molecules on surfaces. The accurate description of dispersion, which often occurs in conjunction with hydrogen bonds, is a major challenge for many electronic structure theories. Density functional theory (DFT), the most widely used electronic structure theory, often doesn't meet this challenge. Many schemes have been developed that allow dispersion to be accounted for within DFT in a more or less approximate manner. One of the most promising and rigorous method is the nonlocal van der Waals density functional (vdW-DF) proposed by Langreth and Lundqvist and co-workers (M. Dion et al., Phys. Rev. Lett. 92, 246401, 2004). We have been working on developing improved versions on the vdw-DF approach, and in particular have developed optB88-vdW, optPBE-vdW, and optB86b-vdW functionals. Some references (more) 1) J. Klimeš, D. R. Bowler and A. Michaelides Van der Waals density functionals applied to solids Phys. Rev. B 83, 195131 (2011) 2) J. Klimeš, D. R. Bowler and A. Michaelides A critical assessment of theoretical methods for finding reaction pathways and transition states of surface processes J. Phys.: Condensed Matter. 22, 074203 (2010)

Quantum nuclear effects

Quantum nuclear effects are important but generally poorly understood. For this reason, we are working to understand them by developing and applying state-of-the art path integral techniques. The types of systems we are looking at include processes at surfaces(chemical reactions and adsorption) and on the fundamental nature of the hydrogen bond. Read more about Quantum Nucelar effects. Some references (more) 1) X. Z. Li, B. Walker and A. Michaelides Quantum nature of the hydrogen bond Proc. Natl. Acad. Sci. USA 108, 6369 (2011) 2) X. Z. Li, M. I. J. Probert, A. Alavi, and A. Michaelides Quantum nature of the proton in water-hydroxyl overlayers on metal surfaces Phys. Rev. Lett. 104, 066102 (2010)