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Title: Emergence in practice : case studies using density functional theory
Author: Schoonmaker, Robert Timothy
ISNI:       0000 0004 7652 2139
Awarding Body: Durham University
Current Institution: Durham University
Date of Award: 2018
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Emergence is a philsophical concept that has proved to be attractive and long lasting. However in some forms, theories of emergence can be at odds with the process of deductive scientific research. Here I develop a theory of historical emergence based on our inability to describe, and therefore explain highly complex physical systems. To provide evidence for this hypothesis, I perform electronic struc- ture calculations on cyclobutadiene, iron arsenide, elemental iron, and manganese oxide using DFT. I find that only in the iron calculations was historical emergence found. I conclude that historical emergence is an effective definition of emergence, as only the iron calculations exhibited all the behaviour expected in a system that hosts emergence, namely dependent novelty, irreducibility, and unpredictability. Further, I propose a general theory that is able to calculate the wavefunction of the nuclei in an effective potential. I use this to calculate the Raman spectrum of cyclobutadiene, in which an energy splitting of vibrational energy levels is found due to tunnelling between two chemically equivalent rectangular configurations. I find that the structure of this spectrum, including the tunnelling splitting, can be explained by recourse to the typical motions predicted from a semiclassical model. I conclude that the properties different isomers cannot be calculated using a single generalised quantum calculation, even though isomers are composed of the same particles and have the same Hamiltonian. Therefore chemical systems are likely to host historically emergent explanations. From the analysis of single-crystal XRES measurements on FeAs, and symmetry considerations I propose a new canted magnetic structure commensurate with the incommensurate elliptical helical magnetic order. I justify this with an orbital projection method that is able to calculate the susceptibilities of the material to spin-orbit interactions. I also detail a spin initialisation procedure based on rotations of the exchange- correlation potential, that aims to reduce bias towards undesired density config- urations by the density search algorithms in noncollinear systems. I present its application to symmetry unconstrained, noncollinear calculations of manganese ox- ide and elemental iron. I conclude that this procedure is not suitable for systems with magnetic configurations robust to changes in their exchange and correlation potentials. Additionally symmetry unconstrained calculations are nontrivial, and future calculations will require modified density search algorithms to deal with sym- metry unconstrained calculations on many conductors due to complex interactions at the Fermi surface.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available