Homogeneous catalysts have transformed synthetic organic chemistry and many of the most popular chemical reactions require a catalyst , and catalyst discovery is one of the "holy grails" of computational chemistry. Quantum chemistry (QM) has become an important tool in elucidating catalytic mechanisms and experimental mechanistic studies frequently include a modelling component. QM calculations can also be used to test ideas for new catalysts that arise from these mechanistic insights. While quantitative predictions of reaction rates and yields are difficult the QM calculations often serve as a "sanity check" for new ideas based on chemical understanding.