Ab initio density functional theory

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 Kohn-Sham determinant and a partial infinite-order resummation of one-body terms was proposed as a solution. The new functional offers a more balanced description of correlation effects, as was demonstrated in applications to a number of closed-shell atoms and molecules. It resulted in energies and densities superior to conventional (Moller-Plesset) second-order perturbation theory or DFT methods, accurately reproduced potential energy surfaces, and led to qualitatively correct effective potentials and single-electron spectra.; An extension of the method based on mixing exact local and nonlocal exchange and an approximate second-order correlation potential were also examined.