Time-averaged electron density

In this case, the phase problem reduces to a question of the sign of the structure factor. Since the sign of the structure factor can be evaluated more reliably than the phase angle, the electron density in a centrosymmetric crystal can be evaluated more accurately than that in an acentric crystal (93), Equation (2) implies that total, time-averaged electron density can be determined by the diffraction method. 

Fig. 2.11 The nuclei in a molecule see a time-averaged electron cloud. The nuclei vibrate about equilibrium points which define the molecular geometry this geometry can be expressed simply as the nuclear Cartesian coordinates, or alternatively as bond lengths and angles r and a here) and dihedrals, i.e. as internal coordinates. As far as size goes, the experimentally determined van der Waals surface encloses about 98% of the electron density of a molecule...

The observed NMR shift, expressed as Am/Fermi contact hyperfine interaction in materials containing 3 /-transition metals is proportional to the unpaired electron spin density at the nucleus site. The magnitude of the interaction is directly proportional to the Fermi constant Ac and the time-averaged electron spin (5z) by... 

Temporary dipoles are caused by the temporary and random fluctuations in the electron density in a molecule or an atom. Over an averaged period of time, the electron density is spread evenly around the nucleus or nuclei. However, at a given instant, the electron density distribution may be asymmetrical, giving the atom or molecule a temporary dipole (Figure 4 69). A dipole is a separation of charge. 

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