2.1 QM/MM energy partitioning QM/MM is based on the partitioning of the full target system into a QM region (A) including the interesting part of the system, such as the active center of an enzyme, and an MM region (B) containing all other atoms, i.e., the active center's environment. The total energy can be expressed as the sum of the QM energy of subsystem A, E QM (A), the MM energy of subsystem B, E MM (B), and an interaction energy between the two subsystems, E int (A, B),