School of Chemistry - Theses

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Start date: 2020 × End date: 2022 × Subject: Density functional theory × Author: Najibi, Asim ×

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    Finding the Best Ingredients of Density Functional Approximations for Ground-state Molecular Chemistry
    Najibi, Asim ( 2021)
    This thesis involves the analysis of the form, efficiency and applicability of leading density functional approximations (DFAs) for modelling ground-state molecular chemistry. Firstly, the DFT-NL methodology of including a density-based nonlocal correlation term to an exchange-correlation density functional is shown to be beneficial for many DFAs. It is also shown that the nonlocal term can be computed in one step after the self-consistent-field (SCF) procedure without loss of accuracy. The assessment in this study is based on the GMTKN55 database of general main group thermochemistry, kinetics and noncovalent interactions. Special emphasis is placed on the B97M-V, wB97X-V and wB97M-V DFT-NL, which I show to be very accurate and robust functionals. Secondly, the use of the DFT-D4 dispersion correction is investigated. This is based on the geometry, charge and electronic spin of a molecule. In addition to energetics, I investigate the approach for geometry optimisations. Several DFT-D4 corrected functionals are compared to their DFT-D3 dispersion corrected counterparts, including B97M-V, wB97X-V and wB97M-V, where the nonlocal density-based term is replaced by the DFT-D3/D4 correction. The DFT-D3 correction was developed prior to the DFT-D4 correction, and is not dependent on the charge and electronic spin of the molecule. Hence, there is emphasis on the two approaches for transition-metal systems. The DFT-D4 correction can indeed be more accurate than the DFT-D3 correction for charged and open-shell systems. Lastly, various range-separated double-hybrid functionals are assessed for energetics, utilising the GMTKN55 database. These methods calculate the exchange energy in a short-range regime composed of DFT and Fock exchange and a long-range regime composed of only Fock exchange. This is done with the aim of overcoming various problems inherent to DFAs, such as the self-interaction error, in which interactions of electrons with themselves are mistakenly calculated. This study investigates the wB2PLYP and wB2GP-PLYP semi-empirically-parameterised range-separated double hybrids, and the RSX-0DH and RSX-QIDH non-empirically-parameterised range-separated double hybrids.