where |ψ G ⟩ is the lowest spin-singlet eigenstate and ψσ (r) is the field operator. One should note that this density when integrated and summed over spin directions (σ ± 1) is equal to the total number of particles (N e = 2). Obviously, n ↑ (r) = n ↓ (r) ≡ n(r)/2, where n(r) is the total density. More importantly, this quantity provides the physical density, in contrast to the probability density |ψ G (r 1 , r 2 )| 2 . This distinctive feature of n(r) shows that the density diminishes in the region between the atoms to zero relatively fast with the increasing R above R M ott . To substantiate the last statement we have listed in Tab. 2 the density value n(0, 0) in the middle point between the proton positions. For the sake of comparison, we have also added there the inverse orbital size, as well as the mixing coefficients in the wave function w i (r) = w(r -R i ) to show, that indeed both two-and single-particle characteristics merge into their atomic correspondants as R increases beyond R M ott . Note that whereas γ expresses the decreasing Pauling covalency, 12 n(0, 0) describes the diminishing true covalency. Analogously, we can define the concentration of the electrons in ionic configuration as follows: