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where ${\bf E}_{tot}$ is the total electric field, ${\bf E}_{ext}$  is the external (or driving) electric field, and $\left\langle \cdots\right\rangle $  indicates a time average.   Large field enhancements are the key to single molecule surface-enhanced  Raman spectroscopy (SERS) and values as large as $h>100$ (the  intensity of the SERS signal scales as $h^{4}$) are predicted by  classical electromagnetic calculations.\cite{Kneipp_2002}  In classical calculations the electronic response is model by the  macroscopic permittivity of the material. The classical Drude model  gives the following simple and robust approximation of the metal (complex)