Correlations Between Electrons and Nuclei

: From Fast-Ion Conduction to Nonadiabatic Processes

Abstract

Many chemical systems exhibit behaviour that is directly attributable to interactions between the electronic and nuclear subsystems. As such, accurate description of the nature of electron-nuclear interactions is critical for the theoretical investigation of a wide range of chemical phenomena. This thesis presents two strands of research that focus on different manifestations of electron-nuclear correlation in quantum chemistry: fast-ion conduction in solid electrolytes, and nonadiabatic processes in molecular systems.

The mechanisms of fast-ion conduction in Ba₂ScHO₃ and Bi₄V₂O₁₁ were investigated using multifaceted approaches comprising static and dynamic periodic DFT calculations. For Ba₂ScHO₃, the key H⁻ migration pathways were determined, and the effects of disorder, pressure and cation substitution on hydride mobility were examined. It was found that hydride mobility was reduced by H⁻/O²⁻ disorder, the dominant mechanism changes under elevated pressure, and substituting Ba with Sr substantially increases hydride conductivity. For Bi₄V₂O₁₁, the ionic conductivity was investigated by division of the energy landscape into distinct configurations based on oxide vacancy distributions. The mechanism of oxide migration was determined using AIMD simulations, which highlighted the pivotal role of the vanadium ion environments in oxide conduction.

A novel method to calculate rate constants of internal conversion (IC) in molecular systems was developed, implemented and tested. The method, NASCF+CF, combines a thermal vibration correlation function formalism with a recently developed nonadiabatic electronic structure method (NA-SCF) to calculate internal conversion rates between electronic states from static calculations at the ground state equilibrium geometry. A novel derivation of the correlation function in the occupation number representation is presented, and the recently proposed one-shot method for calculation of vibronic coupling matrix elements within the NA-SCF method was implemented. When applied to a series of azulene-based molecules, NASCF+CF performed favourably compared to similar computational methods and generated inter-excited state IC rates in qualitative agreement with experiment.

Date of Award	19 Mar 2024
Original language	English
Awarding Institution	University of Bristol
Sponsors	Centre for Doctoral Training in Theory and Modelling in Chemical Sciences (TMCS)
Supervisor	Neil L Allan (Supervisor), Frederick R Manby (Supervisor) & Basile F E Curchod (Supervisor)