The use of electric field gradient calculations in charge density refinements. I. Hirshfeld-type deformation functions and the calculation of electric field gradients by Fourier series

Single-crystal X-ray diffraction data contain in principle complete information on the spatial distribution of the bonding electrons. However, this information is subject to a variety of errors. Nuclear quadrupole resonance spectroscopy (NQR) provides a very sensitive measurement of an electrostatic property of the deformation density; the electric field gradient tensor at the site of a nucleus. It is proposed that diffraction and NQR data are combined so that more reliable charge densities can be obtained. Well known multipole deformation functions are used to describe a quasistatic and parametrized deformation density. The electric field gradient is computed by a Fourier series and expressed as a function of the same deformation parameters. These can then be refined by least-squares calculations simultaneously with respect to diffraction intensities and NQR results. Overlap corrections of the neutral spherical atoms and the effect of temperature vibrations are discussed.