H2 + Pt(211) Surface Reaction
This ongoing project is based largely on the work of Roar Olsen when he was working in the Baerends group. He developed the first complete DFT-based Potential Energy Surface (PES) for a molecule interacting with a stepped surface. The PES he developed is state-of-the-art, with an RMS error of 0.02 eV in the energy-range of interest for reaction dynamics.
When the PES was finished, I began a classical trajectory study of the reaction dynamics for the H2molecules initially in the rovibrational ground state. Rather than simply calculating reaction probabilities and probablilities for inelastic scattering, a new tool was developed to allow for a much more insightful study of the dynamics: Dynasity.
Dynasity is a tool for visualizing the time-dependence of systems in 3D. It was used in this case to examine changes in the density of the gas of classical molecules incident upon the surface, as a function of time. Other moments of the system were also examined, such as the density of kinetic energy, rotational energy, and translational energy. These studies, when taken together, helped to build up a detailed picture of how, where, and by what mechanism, reaction occurs on the stepped surface.
Work is currently being undertaken to develop software to solve the dynamics quantum mechanically. A new tool, Periphery, is being developed in C++, utilizing contemporary software development techniques. It is hoped that this tool will be much more maintainable and extendible than the Fortran programs developed in the past, and will have a long and useful life.
OH + CO → CO2 + H Gas-Phase Reaction
Prior to leaving Leiden University, where I was based for around 3 years as a Postdoc under the leadership of Geert-Jan Kroes, I undertook a difficult challenge: calculating quantum mechanical reaction probabilities for the OH + CO → CO2 + H reaction. This system is particularly challenging for quantum mechanical calculations, because the potential includes a very deep well, demanding large basis sets, and long propagation times in the case of wavepacket methods.
The system turned out to be even harder than could have been anticipated from earlier calculations described in the literature, but eventually, with a large investment of computer resources, the first full-dimensional quantum calculations were undertaken. A new wavepacket program was developed especially for the purpose (quadtk), based on Fortran 90 with OpenMP for parallelization.
This research has shown that the CO bond is basically a spectator, in that it is not necessary to model it accurately to produce reasonably accurate reaction probabilities. Rosendo Valero has inherited this project, and is currently undertaking calculations for vibrationally-excited initial states.
H2 + Cu(100) Surface Reaction
Much of my work in Leiden involved the study of H2 reacting on a Cu(100) surface. I studied this fundamental activated surface reaction for several years, performing classical, quantum, and semi-classical calculations for it.
Quantum studies were performed using the Symmetry-Adapted Wave Packet (SAWP) method. I extended the existing implementation so that a wider variety of initial states could be studied. New software, called CAST, was written for the classical and semi-classical studies.
One of the more interesting results to come out of this research was a hypothetical technique that I christened ‘Molecular Knifethrowing’. Research revealed that the active reaction site could be strongly dependent on the initial rovibrational state of the H2 molecules. This implies that by preparing an appropriate initial state, molecules could be made to selectively react at different reaction sites on a surface. The analogy with knifethrowing, which also involves preparing an initial state in order to target a site, seemed natural.
Preservation of Zero-Point Energies in Classical Trajectory Simulations
Classical trajectory calculations sometimes exhibit a problem known as zero-point energy leak. Basically, a classical system can access its zero-point energy, using it for barrier crossing, for example, where a quantum system is required to preserve it. This can lead quasi-classical calculations to overestimate reaction probabilities and rates.
My PhD research involved testing different algorithms for preserving zero-point energy in classical and quasi-classical calculations, in an effort to prevent zero-point energy leak. The research succeeded in developing methods capable of zero-point energy preservation, though these methods were shown to change the dynamical nature of some systems, which is an undesirable side-effect.
Forces Between Colloidal Particles
My first research project was undertaken in my honours year, in the Applied Mathematics group at Melbourne University. I was charged with writing a program to calculate the forces between two plates separated by electrolyte. These calculations could be used to approximate the force between two colloidal particles.
The program I wrote led to my first publication, and is still used in the Chemistry Department at Melbourne University, where it makes predictions for experiments measuring the force between colloidal particles.