View source for Multipole density formalism


==Definition==

The multipole density formalism is a way to  improve the modelling of the electron density of crystal structures by departing from the purely spherical and independent atom model. In this formalism, the electron density  of each atom is  split into two parts, the core and the valence electron densities.  The  density of the valence electrons is again split into two parts, namely a spherical and an aspherical density. The aspherical density  is based on spherical harmonic functions which are usually limited to the lowest orders.
The aspherical part is thus able to improve the modelling of ''e.g.'' valence bonds  in organic molecules or electron pairs  around some metal atoms.

==Bibliography==

Coppens, P.; Hansen, N. K. (1978). "Testing aspherical atom refinements on small molecule data sets" . Acta Cryst.. A34 (6): 909–921. https://doi.org/10.1107/S0567739478001886

Genoni, A.; Macchi, P. Quantum Crystallography in the Last Decade: Developments and Outlooks. Crystals 2020, 10, 473. https://doi.org/10.3390/cryst10060473