Correlation Effects on Ionization Energies. A Comparison of Ab Initio and LDA Results

Abstract

Since the advent of photoelectron spectroscopy (Turner et al., 1970; Siegbahn et al., 1971) the interest in atonic and molecular ionization energies has remained very high. At a first approximation the experimental data can be interpreted as a measurement of orbital energies, making therefore a direct connection with orbital theories of electronic structure (Siegbahn and Karlsson, 1982). Notably, in the abinitio framework, the most immediate connection is with the eigenvalues of the Fock operator, through the Koopmans theorem (KT). Within a single determinant approach a refined description may be achieved by reoptimizing orbitals for each final ionic state, the so called ⊿SCF calculation. The energy gained in the ionic states, termed relaxation energy, lowers the calculated IEs, and is particularly important for the deeper hole states, although it may be very significant also in the valence shell, typically in transition metal compounds. It has been soon realized however that important deviations, notably inversions in the ordering of the ionic states, are sometimes obtained both at the KT and ⊿SCF levels. Also additional states become accessible, corresponding to multiple electron excitations, which are forbidden at the KT level, and can be only partly explained by the ⊿SCF approach through the loss of orbital orthogonality.