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Intermolecular interaction and solvent effects play important roles in determining physical and chemical properties of molecular systems, and must be considered in relevant quantum mechanical (QM) calculations. Due to the high computational cost, full and rigorous QM treatment of both solute and solvent molecules is impractical. Computationally efficient molecular mechanical (MM) methods can be used to describe solvent effects, and combined into QM methods to formulate QM/MM methods. Classical force field method and reaction-field method are the two most popular MM methods. However, the issue of effectively combining MM methods with high-level QM methods remains unsolved. This thesis presents several novel QM/MM methods. The first is a heterogeneous reaction-field method that can be used to study solute molecules at the interface between two or more phases characterized by different dielectric constants. The second is a second-order perturbation theory/reaction-field method that can be used to obtain accurate QM results in the presence of a reaction-field for both close-shell and open-shell molecules. The third is a time-dependent density functional theory/polarizable force field method that can be used to study solvent effects in electronic transition and excited state molecules.