AB INITIO DENSITY FUNCTIONAL THEORY By IGOR VITALYEVICH SCHWEIGERT A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA

of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy AB INITIO DENSITY FUNCTIONAL THEORY By Igor Vitalyevich Schweigert August 2005 Chair: Rodney J. Bartlett Major Department: Chemistry Ab initio Density Functional Theory (DFT) is a new approach to the electronic structure problem that combines elements of both density-functional and wavefunction-based approaches. It avoids the limitations of conventional DFT methods by using orbital-dependent functionals based on the systematic approximations of wavefunction theory. The starting point of ab initio DFT is the exact exchange functional. This functional was implemented with the auxiliary-basis Optimized Effective Potential method. The effect of numerical parameters on the performance of the method was also examined. It has been suggested in the literature to use perturbation theory to construct the correlation counterpart of the exact exchange functional. In this study, an ab initio correlation functional from second-order perturbation theory was implemented. However, numerical tests showed that this functional fails to provide an adequate description of correlation effects in molecules. This problem was attributed to the poor convergence of the perturbation series based on the KohnSham determinant and a partial infinite-order resummation of one-body terms was