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$\times 1e=4.80320$D.

References

Related reading

• Carlos Vega "Water: one molecule, two surfaces, one mistake" Molecular Physics 113 pp. 1145-1163 (2015)