Dipole moment

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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, 1D 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 = 2.9979 10^{29}D. This is unit is convenient for molecular systems since 1Å\times 1 e=4.803D.