Using quantum mechanics on larger systems with high level methods is very demading in terms of computer time and is therefore normally out of the question. What seems to be the best alternative at the time being, is to combine high level approximations on the interesting part of your system (the active part) with some appropriate low level approximation on the rest of the system (the nonactive part).
Methods trying to combine MO (molecular orbital) approximations describing the active part of the system (ab initio, density functional to semi-empirical) with either some lower level MO methods or MM (molecular mechanics) describing the nonactive parts, will be discussed. These are:
With this method you treat the active part of your system with MO methods and the nonactive part with MM.
With this method you treat the active part of your system with sophisticated MO method, whereas the nonactive part is treated with some lower level MO method.
The ONIOM method divides the system into n layeres like an onion. What we will discuss is the ONIOM3 method, that divides your system into 3 parts. With ONIOM3 you can use high level MO methods to describe the active part, some lower level MO method to the semiactive part and MM to the nonactive part of the system. An example could be CCSD(T) on the active part, HF or MP2 on the semiactive part and MM on the nonactive part of the system.