              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.