Mercury clusters from van der Waals to the metallic solid : a thesis presented in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Theoretical Chemistry at Massey University, Albany, New Zealand
The nature of mercury clusters is studied in an attempt to reconcile the behaviour of the solid with that of the smallest molecules. Related systems such as Zn, Cd, and Ba are investigated for comparison. A range of ab initio methods are employed, and their accuracy assessed. Density functional theory (DFT) based methods are shown to be unreliable. Different functionals vary widely in their description of a particular system, such as the dimer, while individual functionals vary in accuracy when applied across a range of system sizes. This is related to the neglect of van der Waals forces by DFT for the smaller systems, but raises interesting questions about the solid. Wavefunction-based methods are seen to be much more reliable than DFT, although a high-level description of correlation is required. Hartree-Fock (HF) calculations are shown to be consistent in their description of systems of all sizes, and therefore although inadequate on its own the addition of a correlation potential derived from the many-body perturbational (MP2) calculation for the dimer corrects HF to produce exactly the correct bond lengths (when compared to the best known data) for all sizes up to the bulk lattice. The use of higher order many-body potentials is investigated and compared to the situation observed for the noble gases, since for small sizes these are the closest analogues of the neutral mercury clusters. The question of how to simulate transitions in large clusters is addressed. Transitions of interest in clusters are the liquid to solid phase transition, the metal to non-metal transition, or a structural transition from one isomeric motif to another. Therefore the ability to calculate the properties of these clusters accurately is as important as the question of structure. Four-component DFT calculations for the mercury dimer polarisability agree well with the anisotropy derived from Raman spectroscopy. Various isomers proposed in the literature are compared for the smaller mercury clusters. The structures of cationic clusters are also optimised, and their electronic excitation spectra are investigated through CIS(D) and TD-DFT calculations and compared to experimental results. The structures of anionic zinc clusters are obtained and the density of states compared with experiment. The structures and spectra of these clusters are related to those seen for the magnesium analogues, and the effect of the d-electrons in perturbing the jellium model description of these clusters is considered.