Let us now let RealQM explain why two Helium He atoms do not bond to a He2 molecule, while a lithium cation Li+ forms a molecule with the anion H- with strong bond. The set up is thus
- Each He has a +2 kernel surrounded by 2 electrons.
- Li+ has a +3 kernel surrounded by 2 electrons and H- has +1 kernel surrounded by 2 electrons.
The essential difference is thus the two +2 kernels of He2 to be compared with the +3 and +1 kernels of Li+H- with 4 electrons in both cases.
As concerns He2 we refer to
this post explaining the lack bond of He2 as a consequence of an outward shift of the electron charge distribution counteracting the bonding effect from accumulation of charge between the kernels.
We now compare the He2 computation with corresponding computation for Li+H- with thus a change from +2 and +2 kernel charge to +3 and +1 and get the following result
running this code:
We obtain a ground state energy of Li+H- = -8.08 Hartree to be compared with the energy of Li+ = -7.28 and H- = -0.527 altogether = -7.807, which indicates a dissociation energy of Li+H- = 0.27. According to chatGPT this matches the value 0.26 by FCI and CCSD(T) as best value using Standard Quantum Mechanics StdQM.
RealQM and StdQM thus give the same dissociation energy for Li+H- but from different mathematical models: RealQM is based on non-overlapping one-electron charge densities as problem in 3 space dimensions, while StdQM requires 12 space dimensions for the 4 electrons involved. RealQM has a direct deterministic physical meaning, while the physical meaning of StdQM is still an open problem.
The advantage of RealQM is that an explanation of the binding of Li+H- can be read from the above 2d section through the kernels of Li+ and H-, and the 1d section of the one-electron wave functions in yellow:
- We see to the left 2 half-spherical electron/wave function distributions around the +3 kernel meeting at a free boundary represented in the meeting of yellow curves.
- Similarly we see to the right 2 half-spherical electron distributions around the +1 kernel meeting at a free boundary.
- We see an accumulation of charge between the kernels with wave functions meeting at a free boundary, which creates a bond because the accumulation does not require increase of kinetic energy since the wave functions meet with non-zero common value.
- We see that the free boundary for H- is shifted to the right which decreases the presence of the left half-spherical electron between the kernels and so decreases the bond.
- The effect of 3 shows to dominate over 4 and so forms a strong bond.
We now compare with the finding of
this post that the Helium atom He does not form a molecule He2 since there is no binding, because in this case the effect of 3 is cancelled by 4 to no bond.
RealQM offers a concrete physical explanation of both the strong bond of Li+H- and the no bond of He2.
RealQM agrees with StdQM as concerns dissociation energies in both cases.
StdQM does not offer any physical explanation because it is based on a non-physical model.
We see also that even if the Li+H- is viewed to have an ionic bond, because an electron has been transferred for Li to H, the actual bond between Li+ and H- is formed as a covalent bond from electron accumulation between the kernels, or "sharing electrons" in StdQM terminology. It suggest that ultimately all chemical bonds are covalent. In particular Li+ participates in bonding and the StdQM idea that it cannot because its two electrons form a "filled shell", does not seem to represent physics.