However, such approaches based solely on distance criteria or the electrostatic effect might miss residues that play an important steric role or take part in covalent interactions along allosteric pathways in enzymes. This is particularly relevant if the structure of the active site changes along the reaction path, which is not captured by a purely electrostatic analysis. Hix et al. used an analysis of evolutionary conserved residues in protein families to identify such residues and to systematically construct QM regions based on this information . However, this approach relies on the availability of protein sequence data for families of proteins that are related to a system of interest.