Electron Density, Density Matrices and Atomic Properties

Abstract

Though the focus of the volume is on correlation effects in the multicentre problems of molecules and condensed phases, it will be helpful to start by considering the single-centre problem of atoms. A brief discussion of gross trends of atomic ground-state energy with increasing number of electrons serves already to highlight the importance of the electron density in many-body theory—a recurring theme through this volume. It also prompts the generalization of electron density to its off-diagonal form, the first-order density matrix. This latter quantity determines the kinetic energy, of an atom say, but one needs the second-order density matrix to yield the total ground-state energy (see Eq. (2.21) below). Collective effects are then briefly considered; and the Chapter concludes with reference to (i) d-dimensional theory and (ii) the coupled cluster method.