In the last several decades, plasmonic metal nanoparticles (MNPs) have generated a great amount of interest due to the dramatic effect of surface localized plasmon resonance. Currently, plasmonic MNPs have been widely used to control and manipulate light at the nanoscale, thus regulating the photophysical and photochemical properties of molecules in their vicinity, such as enhancing molecular optical signals (absorption, Raman, and fluorescence ) and mediating molecular photochemical reactions. Meanwhile, many theoretical and computational methods were developed to unravel the detailed mechanisms of molecular spectral enhancements, plasmon-mediated chemical reactions, as well as plasmon-enhanced resonance energy transfer. In many cases, the plasmon enhancement arises mainly from the electromagnetic mechanism, where an external electric field can be substantially magnified by an MNP around its surface. Accordingly, several classical nonatomistic methods had been widely applied.