Nuclear quadrupole moment ($Q=\int \rho (\mathbf{r})(3z^2-r^2)d^{3}r$)

The electric quadrupole moment of an atomic nucleus, conventionally specified in its stretched nuclear spin state. It measures a nonspherical nuclear charge distribution. Its interaction with the electronic electric-field gradient contributes to hyperfine structure and generally changes the magnetic-dipole Landé interval rule. A permanent rank-two moment vanishes for nuclear spin less than one.