Polarisability

When a molecule is in an electric field, its distribution of charge is displaced. This induces a dipole μ. The polarisability α relates this dipole to the electric field:

\[\mu = \alpha E\]

As a simple rule of thumb, the polarisability is proportional to the volume occupied by the molecule.

The polarisability may be anisotropic. It can be represented by a polarisability ellipsoid, which is a 3D surface. The distance of a point (i) on the surface from the molecule’s electrical centre is inversely proportional to the square root of the polarisability along the line from that point to the centre.

\[{r_i} \propto \frac{1}{{\sqrt {{\alpha _i}} }}\]