  Standard X-ray crystallography methods use free-atom models to calculate mean
unit cell charge densities. Real molecules, however, have shared charge that is
not captured accurately using free-atom models. To address this limitation, a
charge density model of crystalline urea was calculated using high-level
quantum theory and was refined against publicly available ultra high-resolution
experimental Bragg data, including the effects of atomic displacement
parameters. The resulting quantum crystallographic model was compared to models
obtained using spherical atom or multipole methods. Despite using only the same
number of free parameters as the spherical atom model, the agreement of the
quantum model with the data is comparable to the multipole model. The static,
theoretical crystalline charge density of the quantum model is distinct from
the multipole model, indicating the quantum model provides substantially new
information. Hydrogen thermal ellipsoids in the quantum model were very similar
to those obtained using neutron crystallography, indicating that quantum
crystallography can increase the accuracy of the X-ray crystallographic atomic
displacement parameters. The results demonstrate the feasibility and benefits
of integrating fully periodic quantum charge density calculations into ultra
high-resolution X-ray crystallographic model building and refinement.
