Computational chemistry has become important in organic synthesis as it provides a detailed understanding of molecular structures and properties and detailed reaction mechanisms. Besides mechanistic verifications, computational techniques can be used as complementary tools to predict reagents that make reactions happen based on calculated reaction profiles. Moreover, new catalyst design processes can be accelerated by incorporating these theoretical techniques into the searching protocols. Currently, advanced computer technology enables the fast development of highly accurate computational applications in complex molecular systems at a lower cost. In this review, we highlighted examples of recent applications that demonstrate the advantages, limitations, and solutions of these techniques, especially in multiscale approaches.