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Title: Density matrix renormalisation group calculations of the electronic structure of conjugated molecules
Author: Paiboonvorachat, Nattapong
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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With many interesting electronic and optical properties, conjugated molecules have long been a subject for both theoretical and experimental studies, especially in recent applications in optoelectronics. Most of the properties are controlled by their electronic structures, with the low-lying states predominantly characterised by the 7!'-electrons. We have thus employed the semi-empirical Pariser-Parr-Pople model, whose Hamiltonian parameters are optimised against the excited states, in this study. Despite only considering the π-delocalised system, the model has been widely and successfully applied to study such molecules, giving fairly accurate results. The calculation of these strongly correlated systems requires an accurate computational method, which is usually limited to small molecules due to an enormous amount of resources demanded, as in ab initio calculations. Density Matrix Renormalisation Group (DMRG) provides a systematic way to truncate the size of the Hilbert space while being able to reasonably capture the correlation effect. We have compared three different approaches using both localised and delocalised basis states in the DMRG calculations. First, in the real-space method, atomic sites are aligned on the one-dimensional DMRG lattice as originally developed to study the fermionic systems in condensed matter physics. Alternatively, a collection of atoms, e.g. a monomer unit in conjugated polymers, can be used where an in situ optimisation technique is employed to pre-select the important states. Last, the correlation effect between the Hartree-Fock (HF) molecular orbitals on the lattice sites is calculated in the HF -DMRG scheme. In this post-HF method, the energy converges more slowly than the first but the implementation of t he codes is generic to any geometric structures. In addition to the spin-flip and particle-hole symmetries, which are available in the real-space method, the spatial symmetries can also be applied to target specific states when working in HF space. Calculations have been made on conjugated polymers, i.e. polyacetylene and poly(parophenylene), and aromatic compounds, i.e. pentacene and porphin. The results are compared to other methods, e.g. exact diagonalisation and Configurational Interaction Singles, as well as other theoretical and experimental results in the literature
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available