11 Molecules in Complex Environments: Solvent Models, QM/MM and QM/EFP Features, Density Embedding

11.6 Projector-Based Density Embedding

The exact density embedding method adapted from the method of Manby, Miller, and coworkers615 allows embedding calculations to extend beyond electrostatic embedding. This embedding scheme allows for the fragmentation of a system into two interacting subsystems that are treated at two different levels of quantum mechanics (QM/QM), for example coupled cluster embedded in DFT. This type of embedding fully accounts for polarization as well as quantum mechanical exchange, as calculated from the super-molecular embedding density and the exchange correlation functional used. The goal of this embedding theory is to perform a higher-level QM (DFT or WFT) calculation in an environment described by a lower-level QM theory.