Nuclear quantum effects from ab initio simulations : hydrogen-bonds and tunnelling rates

• Main Author: Fang, Wei
• Other Authors: Michaelides, Angelos
• Published: University College London (University of London) 2017
• Subjects: 540
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.747108

The quantum nature of nuclei plays an important role in materials and molecules containing light mass elements, for example in enzymes and water, which are crucial to life and technology. Yet in materials modelling, nuclear quantum effects (NQEs) are often ignored. Using the path integral theory, NQEs in hydrogen bonds and reaction rates for various systems described with density functional theory (DFT) have been examined. A review of the theoretical background, including DFT, path integral molecular dynamics, and the instanton rate theory, is given in Chapter 2. In Chapter 3, the NQEs on the binding energy of DNA base pairs are investigated, revealing that NQEs strengthen the binding of base pairs at room temperature. A counter-intuitive temperature dependence where NQEs are less significant at low temperature was also discovered and explained through a picture of competing quantum effects. In Chapters 4 and 5, hydrogen diffusion on metal surfaces is studied. Calculations show that ``broad-top" diffusion barriers exist on many metal surfaces, and that these have a distinct tunnelling behaviour compared to conventional ``parabolic-top" barriers. The theoretical and experimental implications of this unique behaviour are discussed. In Chapter 5, an improved multidimensional instanton rate theory is tested and applied to hydrogen diffusion on Pd(110), a complex potential energy surface that includes an interesting diffusion pathway over a second order saddle point. Finally, tunnelling assisted water diffusion on Pd(111) is studied using the instanton rate theory combined with DFT. It is found that tunnelling, including the tunnelling of oxygen atoms, plays a role in changing the water dimer diffusion mechanism on this surface at below 40 K. Water monomer and dimer diffusion on a variety of transition metal (111) surfaces have also been studied in this work.