Dipole moment

The electric dipole moment is a measure of polarity. It is the second term in a multipole expansion of a field (the first one being the monopole, or Coulombic, term.) Its definition, for a system of point charges, is given by

$\mathbf{p}=\sum _{i=1}^N{q}_i{\mathbf{r}}_i.$

For molecular systems, it is customarily given in units of Debyes, after the physicist Peter J. W. Debye. In SI units, $1$D equals approximately $3.33564\times 10^{-30}$ Coulomb-meter (exactly $10^{-21}$C m$2^{}$/s divided by $c$, the speed of light in vacuum). Conversely, 1 C m = $2.997910^{29}$D. This is unit is convenient for molecular systems since 1Å$\times 1e=4.803$D.