With advances in nanofabrication combined and increased understanding of plasmonic properties of nanomaterials, SERS has been transformed into a powerful analytic technique for determining structural properties about a system. Although SERS was discovered several decades ago, a complete picture of the enhancement mechanism is very active area of research. With the discovery of single molecule SERS which gives rise to the possibility of 10^10 enhancement factors or more. The total SERS enhancement mechanism involves two contributions resulting, respectively, from electromagnetic (EM) and chemical interactions between the adsorbate and the metal nanoparticle. While it is not possible experimentally to make a clear separation of their individual contributions, from theoretical point there are many ongoing efforts to identify and understand contributions from various possible mechanisms. Although we prefer to discuss these enhancement mechanisms individually, it is important to realize that all these enhancement mechanisms are strongly dependent on the Raman excitation wavelength and will therefore only be important for specific wavelengths.

In the current work, we have simulated the Raman properties of a large set of benzenethiol derivatives, which only differ in their functional groups in the para- and meta-positions. Different functional group on the benzenethiol allows gaining better insight of the chemical interactions between benzenethiol ring and the metal cluster. The experimental results indicate that, halogens at para-position of benzenethiols appear to boost overall chemical enhancement. Herein we present initial results of a systematic study of the CHEM enhancements of benzenethiol derivatives interacting with small silver cluster using time-dependent density functional theory (TDDFT).