The root-mean-square-deviation of the electron density in the
final difference Fourier map. This value is measured with respect
to the arithmetic mean density and is derived from summations
over each grid point in the asymmetric unit of the cell. This
quantity is useful for assessing the significance of the values
of _refine.diff_density_min and _refine.diff_density_max, and
also for defining suitable contour levels.