Calculation of mean excitation energies

Abstract We present a review of calculations of mean excitation energies of small molecules, atoms and their ions using the Random-Phase Approximation. We discuss the relationship with other calculations and we propose several simple rules that may be used to estimate mean excitation energies of cations and singly charged anions. We conclude that the accuracy of the mean excitation energies obtained using this method in terms of choice of basis sets and inclusion of electron correlation is enough to obtain experimental accuracy of stopping powers of gas phase small molecules, atoms and their ions.