DFT and DFT/MM calculations are carried out on the rate-determining step of the addition of dihydrogen to methyl-(N)-acetylaminoacrylate catalyzed by a rhodium catalyst containing a bidentate phosphine-phosphinite ligand. DFT calculations reproduce the experimental results, while DFT/MM calculations do not. The failure of DFT/MM methods for this particular problem is analyzed through a series of calculations with different partitions between the DFT and MM regions, which show that electronic effects of all ligand substituents considered are critical. The analysis of these electronic effects provides key information on the role of each of the substituents in the outcome of the overall catalytic process.