The Generalised Oscillator-Strength Density of the Helium Atom, Calculated by a New Implementation of the Complex-Coordinate Method

Abstract

Abstract We report a calculation of the generalised oscillator-strength density (GOSD) of the helium atom for low momentum transfer. The calculation was performed by extraction of the searched entity from the resolvent of the complex-rotated Hamiltonian. That Hamiltonian was found using a standard ab-initio program without any change in the program code. The shape of the Bethe surface is reproduced (including resonances and the absorption edge). However, with increasing momentum transfer the magnitude of the calculated values for the Bethe ridge becomes too small. This is clearly demonstrated by an evaluation of the Bethe sum as a function of the momentum transfer. A comparison with recent additional theoretical data shows that the deviations are due to the employed basis set.