In the physical and biological systems, most of the reactions take place in the fluid/liquid medium rather than the gas phase. In the coming chapters, we have studied the lithium cluster in the electrolytic media in the perspective of the Li-ion battery system. Therefore, it is necessary to discuss the mathematical model/theory which is used to simulate the atomic/molecular component in the fluid media. These media can be incorporated in the quantum chemical calculation with the proper implementation of the solvent model. In general, the solvent models are the computational technique/method that has been developed to account for the behaviour of the solvated condensed phase. The solvent models allow to simulate the chemical reaction and biological process in the solvated phases with improved understanding and prediction. There are various solvation models which are generally classified into two classes, explicit and implicit models, on basis of the physical appearance of the solvent molecule . Both the models are discussed in the following section.