As attractive and powerful the embedding approaches presented in Sections 3.2 and 3.3 are, the calculations are, in general, still computationally demanding if applied to complex molecular systems. In some situations, it can be attractive to use a subtractive embedding scheme as mentioned in Section 3. Instead of explicitly coupling the quantum and classical regions via electrostatic interactions as in Section 3.3, the QM part is also represented using the same classical model as the environment. One then performs two calculations with the classical model: one using a parametrization of the classical model based on the ground state and one using a parametrization based on the excited state of interest. Then, similar to the ∆-QM/MM-SCF of the previous section, the difference of the classical total energies is an energy correction to the respective vacuum excitation energy, cf. Eq. ( ). This way, the calculation of energy corrections is very fast and can be readily applied to, for instance, large molecular morphologies of amorphous materials, or is easily combined -depending on the details of the microelectrostatics model -with many of the available methods for efficient large-scale electrostatics methods like Particle or Particle-Mesh Ewald .