Exploring the Helium Spectrum and the Helium Polarizability with a Simplified Fully Correlated Approach: Theory and Applications

Because of strong correlation among the electrons, the helium spectrum cannot be described using the Hartree-Fock method to sufficient accuracy. One needs to resort to fully correlated basis sets without the approximation of one-electron orbitals. The fully correlated wave functions typically contain an explicit dependence on the interelectron distance. We explore a simplified approach based on sampling the space of both short- and long-range correlations of the electrons, without the additional need for the optimization of any parameters in the basis beyond the linear expansion coefficients of the trial wave functions inside the chosen basis. The suitability of the method is demonstrated to at least 10 significant figures, the dynamic polarizability is calculated, and atom-surface potentials are analyzed.