Symbol α‎. A measure of the response of a molecule to an external electric field. When a molecule is placed in an external electric field, the displacement of electric charge induces a dipole in the molecule (see electric displacement). If the electric field strength is denoted E and the electrical dipole moment induced by this electric field p, the polarizability α‎ is defined by p=α‎E. To calculate the polarizability from first principles it is necessary to use the quantum mechanics of molecules. However, if regarded as a parameter, the polarizability α‎ provides a link between microscopic and macroscopic theories as in the Clausius–Mossotti equation and the Lorentz–Lorenz equation.