The electronic structure of silicon

A detailed description of the electronic structure of the crystal-bound silicon atom is derived, using recent millielectron level accuracy X-ray structure factors. A 0.5% expansion of the L shell is detected for the first time, in addition to the known ∼6% expansion for the M shell. The breakdown of the rigid-atom approximation for the atomic thermal motion is demonstrated, and strong evidence is presented against an anharmonic term in the effective one-atom potential, in contrast with neutron measurement but in concord with recent X-ray results.