Theoretical aspects of relaxation phenomena accompanying photoionization of core and valence electrons

This chapter contains a brief survey of some fundamental concepts and relationships needed for the theory of electronic structure of many electron systems with special emphasis on Molecular Orbital (MO) theory. Firstly we state the problem: we need to solve the electronic Schrodinger equation for many electron systems. For this we need the appropriate Hamiltonian and a proper form of the wave functions. Having determined these, we solve the Schrodinger equation to a first approximation by the Hartree-Fock method. We find that the concept of density matrices and Slater's rules for evaluation of complicated integrals are valuable tools not only in the Hartree-Fock approximation but generally in MO theory. The LCAO method is introduced to solve the Hartree-Fock equations and to analyse the resulting wave function. The main deficiency of the Hartree-Fock theory - the correlation error is pointed out, and various schemes for overcoming it are briefly discussed. Finally a brief description of the computer program ATMOL 2 used for non-empirical LCAO MO calculations is given.