X-Ray Crystal Structure with Calculated Structures

Comparison of X-Ray Crystal Structure with Calculated Structures 

Recall (Chapter 2) that there are a number of differences between an X-ray structure and the same structure as determined by any other method, experimental or calculated. Depending on the problem at hand, these differences between the same structure determined by two different methods may be quite important, or negligibly so. X-ray structures differ from the others in some important ways. First, the X-ray structure is determined by the positions of the electron densities corresponding to the atoms. All 

Other structural methods that we have discussed depend upon the nuclear positions, and this difference may be important. X-ray structures also suffer from what is called rigid-body motion. This has little effect upon angles bnt can canse bond lengths to artifically appear shortened by 0.005-0.015 A if determined at room temperatnre (less if determined at low temperatures). Vibrational amplitndes and, hence, things like the unit cell size and refractive index are temperatnre dependent. 

---