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 \(\mathbf{p} = \sum_{i=1}^N \, q_i \, \mathbf{r}_i.\)

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