Re: CCL:Are atoms spherical?

[Christoph.van.Wuellen : at : ruhr-uni-bochum.de]

Are atoms spherical?
 For an isolated atom, since the external potential is spherical, the
 "density" has to be spherical as well. For degenerate states, the
 "density" thus is the ensemble density.
 For an atom in an environment, there is no such argument. E.g. the density
 of a hydrogen atom will be deformed by the nearby presence of another
 hydrogen atom -- they form a bond.
 So, if atoms in VanDerWaals situations are nonspherical, this does not
 mean the "atom itself" is non-spherical.
 Nonspherical atoms as the result of a quantum-chemical calculations are a
 different topic, they mostly aris through symmetry breaking, that mostly is
 a defect of the chosen method. If, e.g. a fluorine atom is treated at the
 single-configuration level and if pz is chosen to be singly occupied, then
 the radial wave function of pz will be different from px and py --- no
 spherical density can be obtained even for an ensemble.
 This will change if one goes to a state-averaged calculation.
 Christoph van Wuellen