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This thesis reports the computational studies of the ground state electronic prop erties of AnOX5 n- (An=Pa, n=2 U, n=l Np, n=0 X=F, CI, Br), of C1M(PH3) 2 (M=Cu, Ag, Au, 111 ) and of MX3 (M-La-Lu, Ac-Lr X=H, F, CI, Br, I) us ing Density Functional Theory (DFT) and ab initio methods. Before presenting the results from these studies, the first chapter introduces computational studies of heavy element compounds through a discussion of selected studies taken from the literature. The second chapter introduces the electronic structure methods that were used during the course of this research. The results of this research are presented in chapters 3 to 5. Chapter 3 discusses DFT studies of the inverse trans influence (ITI) of AnOX5 n_ (An=Pa, n=2 U, n=l Np, n=0 X=F, CI, Br). These studies show that the ITI is due mainly to electronic factors at the equilibrium geometry, the 9ai molecular orbital of AnOXs n_, which is 7r* along the An-Xci5 bond, plays a key role in elongating the cis bond. In chapter 4, the metallophilic interactions of the 6d transactinide coinage metal, 111 , are presented. Comparison of the interaction energies of C1M(PH3) 2 (M=Cu, Ag, Au, 111 ), shows that with the exception of MP2, the post-HF (QCISD, CCSD and CCSD(T)) methods calculate that the interaction energies for the four complexes are approximately constant. The final results chapter (chapter 5) examines the influence of the 4f orbitals in determining the extent of pyramidality of LnX3 (Ln=La-Lu X=H, F-I) using the B3PW91 method. This study reports that the 4f orbitals reduce the extent of pyramidality via a reduction in the 5d orbital-based second-order Jahn-Teller distortion. A comparison between LnX3 and AnX3 (An=Ac-Lr X=H, F-I) is also made. The final chapter summarises the research.

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In this thesis high quality electron densities are used to provide insight into density functional theory (DFT) and to improve the quality of DFT calculations. Chapter 1 provides an introduction to ab initio molecular wavefunction calculations with particular emphasis on the Hartree-Fock method. Chapter 2 outlines important concepts in density functional theory (DFT). This includes a discussion of the Zhao, Morrison and Parr (ZMP) method, which is the key to calculating DFT quantities from high quality densities. In Chapter 3, high quality densities are used to gain an understanding of dispersion interactions in the helium dimer. The investigation seeks to understand the correlation potentials associated with a density distortion that gives rise to the correct dispersion forces. Chapter 4 presents a study of response properties using orbitals and eigenvalues determined from high quality densities. Both magnetic and electric properties are considered and comparisons are made with conventional DFT functionals and wavefunction methods. Chapter 5 makes a comparison between Kohn-Sham eigenvalues and related properties, generated both by conventional functionals and from densities. The influence on NMR shielding constants is considered and two approaches to correcting LUMO eigenvalues are presented. In chapter 6, a DFT exchange-correlation functional determined from a lit to high quality densities is applied to study the gauche effect in 2-fluoroethylamine, 2-fluoroethanol and their protonated analogues. Conclusions are presented in Chapter 7.