# Электрическое квадруполь-квадрупольное взаимодействие

As well as magnetic dipole-dipole and exchange interaction occur in crystal, there is a Columb interaction between electrons on different sites — electric quadrupole-quadrupole interaction.  We are talking about Columb interaction exactly between outer electrons of ions, neglecting  interaction of positively charged cores, because of the state of a particle is determinated by outer electrons.

H_{qq} = \frac{e^2}{|\mathbf{r}_{1a} - \mathbf{r}_{2b} - \mathbf{R}_{ab}|}

It has a significant value in research of systems with degenerated states of ions and is able to cause orbital ordering.

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