Electron Shielding: RealQM vs stdQM

A neutral atom with a kernel of positive charge $Z$ surrounded by $Z$ electrons in some (shell) configuration can attract an outside electron thus forming a negatively charged ion at the release of energy, referred to as (negative) electron affinity. 

For example the electron affinity of Lithium (Z=3) with 2 inner-shell electrons and 1 outer-shell electron has an observed electron affinity of - 0.028 Hartree and Fluorine (Z=9) -0.125 Hartree. 

Ok, so if a Lithium atom can attract a negative electron under release of energy, the kernel must exercise some attraction outside the formally neutral atom, which can be thought of as an effective charge $Z_{eff}$ resulting from incomplete shielding of the kernel by the surrounding electrons. This is referred to as electro-negativity as a qualitative property on a certain empirical scale, see post on electro-negativity.

Because of the physical shell structure of RealQM with non-overlapping electrons, RealQM directly informs that the shielding effect of $N$ inner-shell electrons on outer-shell electrons is $0.5\times N$, so that $Z_{eff} = Z-0.5\times N$ and in particular outside the atom $Z_{eff}=0.5\times Z$, which conforms with about 3 times larger affinity for Fluorine compared to Lithium. The precise shielding effect is directly computable by RealQM. 

StdQM does not deliver any theoretical prediction with the excuse that such a thing must be a very very complicated problem.  

What stdQM offers is Slater's empirical rule to calculate the shielding effect of inner electrons on outer electrons in an atom, which e g says that the shielding effect of the 2 inner electrons of Lithium on the outer electron comes with an empirical factor $0.85$ so that $Z_{eff} =3-2\times 0.85 = 1.30$, while for Fluorine Slater gives $Z_{eff}=4.55$. But Slater says nothing about the shielding effect outside the atom.

On the other hand, RealQM says $Z_{eff} = 3-2\times 0.5=2$ for the outer electron of Lithium and $9-0.5*8=5$ for Fluorine, that is a bit less effective shielding than Slater's rule and so more outside attraction available for electron attraction.  

The reason RealQM gives a direct answer to the shielding effect is its physical shell subdivision without the electron overlapping confusing the picture for stdQM. 

For Helium (Z=2) RealQM gives $Z_{eff}=2-0.5\times 2=1$ and so the anion He- can form even if He is a so called noble gas (with an energy release of $0.8$ Hartree according to code), in agreement with observation. Even He2- with additional energy release of $0.5$ Hartree (code) appears to be possible, but realisation may require very special conditions. 

In stdQM a distinction is being made between electron affinity and electronegativity, with electron affinity the energy release when an single atom adds an electron, and electronegativity rather capacity to share electron in a covalent bond. It is not clear that this distinction is essential and does not appear to be so within RealQM.