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Dipole 

[classi] [classj] dipole [sigma_0] [mu]

Udipole = $\displaystyle {\frac{{\mu^2 \sigma_0^3}}{{r^3}}}$$\displaystyle \left[\vphantom{ \hat{\mu}_i \cdot \hat{\mu}_j - 3\left(\hat{\mu}_i \cdot \hat{r}\right)\left(\hat{\mu}_j \cdot \hat{r}\right)}\right.$$\displaystyle \hat{{\mu}}_{i}^{}$⋅$\displaystyle \hat{{\mu}}_{j}^{}$ -3$\displaystyle \left(\vphantom{\hat{\mu}_i \cdot \hat{r}}\right.$$\displaystyle \hat{{\mu}}_{i}^{}$⋅$\displaystyle \hat{{r}}$$\displaystyle \left.\vphantom{\hat{\mu}_i \cdot \hat{r}}\right)$$\displaystyle \left(\vphantom{\hat{\mu}_j \cdot \hat{r}}\right.$$\displaystyle \hat{{\mu}}_{j}^{}$⋅$\displaystyle \hat{{r}}$$\displaystyle \left.\vphantom{\hat{\mu}_j \cdot \hat{r}}\right)$$\displaystyle \left.\vphantom{ \hat{\mu}_i \cdot \hat{\mu}_j - 3\left(\hat{\mu}_i \cdot \hat{r}\right)\left(\hat{\mu}_j \cdot \hat{r}\right)}\right]$ (13)

where μ is the dipole strength, $\hat{{\mu}}_{i}^{}$ is the dipole orientation of the i-th site, r is the distance between sites i and j, and $\hat{{r}}$ is the unit vector pointing between the sites.