My research focusses on understanding quantum mechanical effects in chemical dynamics. Over the years, my group has developed a number of new methods that can be used to reveal these effects, and applied them to a variety of interesting problems in gas phase dynamics, condensed phase dynamics, and spin dynamics. If there is a unifying theme to our work, it is finding ways to avoid the exponential scaling of quantum mechanics so as to be able to address more complicated and interesting problems. This is especially true of our work on condensed phase dynamics, and it has also played a role in our work on spin dynamics.
Gas phase dynamics
Much of our research in the last thirty years has focused on the quantum mechanical description of elementary chemical reactions in the gas phase. We have learned a great deal during this time about the interpretation of transition state spectroscopy experiments, the role of quantum mechanical resonances in hydrogen atom transfer reactions, the significance of the non-adiabatic effects caused by electronic and spin-orbit couplings, the effect of van der Waals forces on chemical reaction dynamics, and the statistical nature of insertion reactions that proceed via deep potential energy wells. The methods and computer programs that we have developed to study these phenomena are now widely used by the chemical reaction dynamics community and by those interested in cold and ultra-cold molecular collisions.
Condensed phase dynamics
In a line of research that began around twenty years ago, we have shown how the standard path integral molecular dynamics (PIMD) method, which has been used since the mid 1980s to compute the exact static equilibrium properties of quantum mechanical systems, can be generalised to calculate approximate real-time quantum correlation functions, and so used to study the role of quantum mechanical zero point energy and tunnelling effects in condensed phase dynamics. The resulting ring-polymer molecular dynamics (RPMD) method is now well established, and it has been used to reveal the importance of nuclear quantum effects in a wide variety of systems, ranging from low-temperature liquids such as para-hydrogen to room-temperature liquid water and aqueous solutions. It is especially well justified for the calculation of chemical reaction rates, for which it has been used extensively. Our latest work in this area has focussed on developing methods that can be used to account for nuclear quantum effects in electronically non-adiabatic reactions, such as electron transfer reactions.
Most recently, we have been inspired by Peter Hore to develop the theory of radical pair recombination reactions. These reactions are relevant to a wide variety of problems, ranging from how the electroluminescence of organic light emitting diodes (OLEDs) changes in the presence of an applied magnetic field to how migratory songbirds detect the direction of the Earth's magnetic field. The quantum mechanical Hamiltonian of a radical pair is straightforward to write down, but the resulting Schrödinger equation is extremely difficult to solve exactly for a radical pair containing a realistic number of hyperfine-coupled nuclear spins. We have developed both semiclassical and quantum mechanical methods that overcome this difficulty, derived the correct quantum master equations that account for radical pair recombination and electron spin relaxation processes, and performed a variety of benchmark radical pair spin dynamics calculations, including a recent exact quantum mechanical calculation of the spin dynamics of a carotenoid-porphyrin-fullerene radical pair that the Hore and Timmel groups have shown to provide a "proof of principle" for the operation of a chemical compass.