Static and Dynamic Correlation Effects in Atoms

Sammanfattning: The thesis considers the effects of electron correlation by studying the two opposite limits of atoms in the ground state and atoms in strong laser pulses. Computational methods and theoretical approaches differ significantly due to the different physical processes that are studied in these two cases. The thesis has an introductory chapter with a brief discussion of the problem of describing electron correlation, in addition to an introduction to the concept of conserving approximations in many-body perturbation theory. The main part of the thesis consists of four research papers, whereof two consider atoms in strong laser pulses and two deal with ground state energy calculations. Considering atoms in strong laser pulses, the problem of including correlation effects is studied by employing a one-dimensional model atom, which allows for an exact solution. While this model differs from the real atom, it is a convenient testing ground for studying the effects of electron correlation, since the characteristic features of the experimental results appear also here. The description is based on solving the time-dependent Schr"odinger equation, exactly or using some simplified form of the two-particle wave function. The ground state energy of atoms is calculated using many-body perturbation theory. Due to computational difficulties, this theory has previously only to a small extent been employed in energy calculations. By using variational energy functionals, the computational effort reduces significantly, and total energies of atoms are obtained at different approximation levels. From these calculations, it has become clearer which physical mechanisms that must be included in total energy calculations.