The point charge model of nuclear quadrupoles:: How and why does it work

The point charge model of nuclear quadrupoles is analyzed beyond the multipole approximation. The effect of the short-range expansion terms is shown to be responsible for the contamination of the calculated electric field gradients. Several regularities observed in earlier numerical calculations are elucidated and their analysis helps to devise proper computational strategies. The use of the point charge nuclear quadrupole model is strongly advocated in approximate relativistic calculations of electric field gradients at heavy nuclei. (C) 2000 American Institute of Physics. [S0021-9606(00)00402-5].