Scattering-factor coefficient a6, used to calculate electron
elastic atomic scattering factors for the defined atom type.
Electron Elastic Scattering Factors Ref:
International Tables for X-ray Crystallography (2006).
Vol. C, Table 4.3.2.2, pp. 282-283.
Cromer_Mann equation Ref:
International Tables for X-ray Crystallography (1974).
Vol. IV, Table 2.2B
or: International Tables for Crystallography (2004). Vol. C,
Tables 6.1.1.4 and 6.1.1.5.