Electrostatic interaction energies of independent aspherical atoms in molecules

Matter may be looked upon as consisting of superimposed independent atoms, which are spatially confined around specific positions by the chemical forces and which are weakly deformed thereby. Independent atoms with degenerate ground states are not fixed to be spherically symmetric as often supposed; it is more convenient for the interpretation of molecular and crystal charge distribution to refer to nonspherical atomic ground-state densities, the orientation of which is also determined by the chemical forces. In accordance with the virial theorem, one half of the quasiclassical electrostatic interaction energy, EE, of superimposed independent atoms is an approximation to the total bond energy, BE. The BE≈EE/2 correlation also improves if oriented nonspherical instead of spherically averaged atoms are superimposed. This correlation does not mean that the bond energy has been explained electrostatically, because one must know the atomic positions (and orientations) in advance. Furthermore, density deformations due to the quantum-mechanical interactions contribute 2–3 eV to BE.