              The effective number of electrons in the crystal unit cell\
               contributing to F(000). It may contain dispersion contributions,\
               and is calculated as\
\
               F(000) = [ sum (f~r~^2^ + f~i~^2^) ]^1/2^\
\
               f~r~ = real part of the scattering factors at theta = 0\
               f~i~ = imaginary part of the scattering factors at theta = 0